Add image-flash, gateware-flash, etc support for pano_logic_g2 target.

[skiphansen]

I also adjusted the flash partitioning as I discovered that I had the wrong erase size before. The minimum erase size is 256MB, not 128MB.

Hopefully the changes to the top level Make file will be useful for other targets.

[skiphansen]

The build for C=vexriscv TC="vivado" P=arty T="base" F=micropython failed. I'm looking into it.

[mithro]

 OBJCOPY  firmware.bin
chmod -x firmware.bin
python -m litex.soc.software.mkmscimg -f --little firmware.bin -o firmware.fbi
make[1]: Leaving directory '/home/travis/build/timvideos/litex-buildenv/build/arty_base_vexriscv/software/firmware'
real	0m4.117s
user	0m3.585s
sys	0m0.560s
python mkimage.py \
	   \
	--override-gateware=build/arty_base_vexriscv//gateware/top.bin \
	--override-bios=build/arty_base_vexriscv//software/bios/bios.bin \
	--override-firmware=build/arty_base_vexriscv//software/micropython/firmware.fbi \
	--output-file=build/arty_base_vexriscv//image-gateware+bios+micropython.bin
Traceback (most recent call last):
  File "mkimage.py", line 261, in <module>
    main()
  File "mkimage.py", line 160, in main
    "Use --override-firmware=none for no firmware." % firmware)
AssertionError: Firmware file 'build/arty_base_vexriscv//software/micropython/firmware.fbi' not found! Use --override-firmware=none for no firmware.
Makefile:233: recipe for target 'build/arty_base_vexriscv//image-gateware+bios+micropython.bin' failed
make: *** [build/arty_base_vexriscv//image-gateware+bios+micropython.bin] Error 1

[mithro]

@mateusz-holenko -- I'll leave you to review and merge.

[skiphansen]

Oh well. The build that failed yesterday is fixed but there's a new failure. I'll look into it ASAP.

[skiphansen]

Please rerun the C=picorv32.minimal TC="vivado" P=arty T="base net" build. It passed here, it's not clear to me what failed ... temp network issue perhaps?

[mithro]

@skiphansen Reran the failing tests.