bootutil: Support for MCUBOOT_BYPASS_KEY_MATCH

This MCUboot configuration option turns off matching of
public key hash, taken from image TLV, against built in public
key. Such verification is not needed when there is only one
key built in as the signature verification will reject
image signed with unknown key anyway.
Enabling the option allows to slightly reduce MCUboot binary
size by removing the code that does the key matching.
Boot time improvement is not really significant.

Signed-off-by: Dominik Ermel <dominik.ermel@nordicsemi.no>