mcuboot incompatible with Nordic QSPI flash driver

[pabigot]

mcuboot does not work with the Nordic QSPI flash API because the flash_area_read_is_empty() extended API that is in:

• mcuboot: boot/zephyr/flash_map_extended.c
• zephyr: subsys/dfu/boot/mcuboot.c

is being invoked to verify that a single byte is uninitialized. The Nordic QSPI flash API requires that all bus transactions be for a multiple of four bytes (this is a hardware limitation).

Consequently the read of an image state field fails, which causes mcuboot to panic.

Patching the call to read 4 bytes and only use the first allows mcuboot to work. However, the proper fix is likely to be to special-case transfers of less than 4 bytes in the QSPI driver.