Only send write flash cmd to STK500v2 programmers if page not empty

[stefanrueger]

... except if it's for a bootloader: they might need to know about 0xff pages if they use an SPM page erase command and therefore memory does not look like NOR-memory.

This is a slight optimisation for all parts (as writing 0xff pages on NOR-memory flash is a NOP), but more importantly, prevents a firmware error to surface for those parts that need to carry out value polling to determine when writing the page has finished on the device.

Fixes #425

Needs testing with affected parts (ATmega8A) and a flash .hex file with an empty page followed by a non-empty page. Also needs testing eeprom and parts that are not affected by Issue #425, eg, ATmega328P.

[mcuee]

First test: it is good to fix #425 for ATmega8A.

C:\work\avr\avrdude_test\avrdude_bin> .\avrdude_pr1273 -c avrisp2 -p m8a -U .\simpletest_issue425_m8a.hex

avrdude_pr1273: AVR device initialized and ready to accept instructions
avrdude_pr1273: device signature = 0x1e9307 (probably m8a)
avrdude_pr1273: Note: flash memory has been specified, an erase cycle will be performed.
                To disable this feature, specify the -D option.
avrdude_pr1273: erasing chip
avrdude_pr1273: reading input file .\simpletest_issue425_m8a.hex for flash
                with 128 bytes in 2 sections within [0, 0xbf]
                using 2 pages and 0 pad bytes
avrdude_pr1273: writing 128 bytes flash ...

Writing | ################################################## | 100% 0.01 s

avrdude_pr1273: 128 bytes of flash written
avrdude_pr1273: verifying flash memory against .\simpletest_issue425_m8a.hex

Reading | ################################################## | 100% 0.03 s

avrdude_pr1273: 128 bytes of flash verified

avrdude_pr1273 done.  Thank you.

C:\work\avr\avrdude_test\avrdude_bin> cat .\simpletest_issue425_m8a.hex
:020000040000FA
:20000000FFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFF00
:20002000FFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFE0
:20008000000000000000000000000000000000000000000000000000000000000000000060
:2000A000000000000000000000000000000000000000000000000000000000000000000040
:00000001FF

[mcuee]

No regression for ATmega328P as well.

C:\work\avr\avrdude_test\avrdude_bin> .\avrdude_pr1273 -c avrisp2 -p m328p -U .\simpletest_issue425_m8a.hex

avrdude_pr1273: AVR device initialized and ready to accept instructions
avrdude_pr1273: device signature = 0x1e950f (probably m328p)
avrdude_pr1273: Note: flash memory has been specified, an erase cycle will be performed.
                To disable this feature, specify the -D option.
avrdude_pr1273: erasing chip
avrdude_pr1273: reading input file .\simpletest_issue425_m8a.hex for flash
                with 128 bytes in 2 sections within [0, 0xbf]
                using 2 pages and 128 pad bytes
avrdude_pr1273: writing 128 bytes flash ...

Writing | ################################################## | 100% 0.11 s

avrdude_pr1273: 128 bytes of flash written
avrdude_pr1273: verifying flash memory against .\simpletest_issue425_m8a.hex

Reading | ################################################## | 100% 0.08 s

avrdude_pr1273: 128 bytes of flash verified

avrdude_pr1273 done.  Thank you.

C:\work\avr\avrdude_test\avrdude_bin> .\avrdude_pr1273 -c avrisp2 -p m328p -U .\ATTOBASICV234_m32-16MHZ-uart_btldr.hex

avrdude_pr1273: AVR device initialized and ready to accept instructions
avrdude_pr1273: device signature = 0x1e950f (probably m328p)
avrdude_pr1273: Note: flash memory has been specified, an erase cycle will be performed.
                To disable this feature, specify the -D option.
avrdude_pr1273: erasing chip
avrdude_pr1273: reading input file .\ATTOBASICV234_m32-16MHZ-uart_btldr.hex for flash
                with 15206 bytes in 4 sections within [0, 0x7fff]
                using 120 pages and 154 pad bytes
avrdude_pr1273: writing 15206 bytes flash ...

Writing | ################################################## | 100% 5.20 s

avrdude_pr1273: 15206 bytes of flash written
avrdude_pr1273: verifying flash memory against .\ATTOBASICV234_m32-16MHZ-uart_btldr.hex

Reading | ################################################## | 100% 4.96 s

avrdude_pr1273: 15206 bytes of flash verified

avrdude_pr1273 done.  Thank you.

[mcuee]

No regression for the wiring (stk500v2 based) bootloader either.

PS C:\work\avr\avrdude_test\avrdude_bin> .\avrdude_pr1273 -c wiring -P COM4 -p m2560 -D
 -U .\simpletest_issue425_m8a.hex


avrdude_pr1273: AVR device initialized and ready to accept instructions
avrdude_pr1273: device signature = 0x1e9801 (probably m2560)
avrdude_pr1273: reading input file .\simpletest_issue425_m8a.hex for flash
                with 128 bytes in 2 sections within [0, 0xbf]
                using 1 page and 128 pad bytes
avrdude_pr1273: writing 128 bytes flash ...

Writing | ################################################## | 100% 0.07 s

avrdude_pr1273: 128 bytes of flash written
avrdude_pr1273: verifying flash memory against .\simpletest_issue425_m8a.hex

Reading | ################################################## | 100% 0.03 s

avrdude_pr1273: 128 bytes of flash verified

avrdude_pr1273 done.  Thank you.

PS C:\work\avr\avrdude_test\avrdude_bin> .\avrdude_pr1273 -c wiring -P COM4 -p m2560 -D
 -U .\hex\blink-mega2560_lext-test.hex

avrdude_pr1273: AVR device initialized and ready to accept instructions
avrdude_pr1273: device signature = 0x1e9801 (probably m2560)
avrdude_pr1273: reading input file .\hex\blink-mega2560_lext-test.hex for flash
                with 1346 bytes in 4 sections within [0, 0x3106d]
                using 7 pages and 446 pad bytes
avrdude_pr1273: writing 1346 bytes flash ...

Writing | ################################################## | 100% 0.31 s

avrdude_pr1273: 1346 bytes of flash written
avrdude_pr1273: verifying flash memory against .\hex\blink-mega2560_lext-test.hex

Reading | ################################################## | 100% 0.15 s

avrdude_pr1273: 1346 bytes of flash verified

avrdude_pr1273 done.  Thank you.

[mcuee]

@stefanrueger
This PR seems to be safe to merge.