enforce ascending writes in bootloader update

[conorpp]

Fix an issue in Solo bootloader where the version check can be bypassed, allowing earlier firmware versions to be programmed on secure models. The fix is to enforce writes on ascending addresses.

[conorpp]

Thanks to @fcremo and @ikkisoft of Doyensec for the security audit and catching this!

[fcremo]

Almost LGTM. With this patch an attacker can't choose freely which bytes get interpreted as the version.
However, I think she can still call BootDone with an uninitialized last_written_app_address, and a valid firmware, maybe allowing to bypass the version check.

To do this, she could flash an older firmware with BootWrite commands, reboot the key, then issue the BootDone command, which would verify the signature correctly and then use an uninitialized pointer to read the firmware version.