sys/riotboot/flashwrite: when invalidating erase checksum as well

[fjmolinas]

Contribution description

This PR addresses two issues with riotboot_flashwrite_invalidate:

• writing 0XAA to invalidate will no always work, e.g. for stm32f1:

In this mode the CPU programs the main Flash memory by performing standard half-word
write operations. The PG bit in the FLASH_CR register must be set. FPEC preliminarily
reads the value at the addressed main Flash memory location and checks that it has been
erased. If not, the program operation is skipped and a warning is issued by the PGERR bit in
FLASH_SR register (the only exception to this is when 0x0000 is programmed. In this case,
the location is correctly programmed to 0x0000 and the PGERR bit is not set). If the
addressed main Flash memory location is write-protected by the FLASH_WRPR register,
the program operation is skipped and a warning is issued by the WRPRTERR bit in the
FLASH_SR register. The end of the program operation is indicated by the EOP bit in the
FLASH_SR register.

Also FLASHPAGE_WRITE_BLOCK_SIZE was not taken into account, so writing to RIOTBOOT_MAGIC might not work.

Discussing offline with @bergzand it was mentioned that the checksum should also be erased.

To be able to do this I added a FLASHPAGE_ERASE_STATE definition for all CPUs supporting flashpage.

Testing procedure

Use #15559 to test, with this PR these test passes, without it fails the invalidation check.

• BOARD=iotlab-m3 make -C tests/riotboot_flashwrite/ flash test-with-config -j3

[fjmolinas]

Should I squash @benpicco?

[benpicco]

Please squash!

[benpicco]

looks like some old commits have sneaked in

[fjmolinas]

I checked that the test is passing rebased on #15559 on nucleo-f103rb

FEATURES_PROVIDED=riotboot BOARD=nucleo-f103rb make -C tests/riotboot_flashwrite/ flash test-with-config

Image Version: 0x60226910
Image start address: 0x08010c00
Header chksum: 0x42247ad5

Starting the shell
>

current-slot

>
> current-slot
Running from slot 1
TEST PASSED

[fjmolinas]

All green @benpicco

[fjmolinas]

thanks for the review @benpicco!