Previously we checked whether the current vendor header and
the new vendor header are the same by comparing the embedded keyset.
What originally looked like a good idea is not that good, because
this disallows us from ever changing the vendor header signing keys
without causing erasure of the storage during the version update.
This commit fixes that by changing the logic to comparing just the
vendor string.
Change of function names is purely cosmetic:
* vendor_keys_hash -> vendor_header_hash
* check_vendor_keys_lock -> check_vendor_header_lock
