H7 MCUs have ECC and writes do not go through to SRAM until 64-bits have
been written (on another location is written). So use 64-bit writes for
the bootloader-state variable so it is committed before the system reset.
As part of this change, the lower byte of the bootloader address in
BL_STATE must now be the magic number 0x5a5 for the state to be valid
(previously this was 0x000 which is not as robust).
Signed-off-by: Damien George <damien@micropython.org>
c99ed8d6fa