This commit fixes issue where empty (unprogrammed) digest slot out of
multiple supported (e.g. 3 for ESP32-C3) could cause issue in
workflow enablement process.
Notes:
1. This issue was applicable for chips supporting "secure-boot-v2"
scheme with multiple digests slots
2. This issue was affecting only manual workflow, where digest of
public was pre-flashed in efuse
3. Change in "flash_encrypt.c" is only for additional safety purpose
As this mode uses the public keys attached to the existing app's signatures to
verify the next app, checking that a signature block is found on boot prevents
the possibility of deploying a non-updatable device from the factory.
