imgtool: Support for built-in keys

[pamin98]

The MCUBOOT_BUILTIN_KEY option does not require the public key or its hash to be part of the TLV section. However, there is currently no option in the imgtool to skip this TLV as the public_key_format argument forces the addition of either a KEYHASH or a PUBKEY TLV. The --public-key-format input argument currently supports 2 options: 'hash' and 'full'. A third option, e.g. 'builtin' could be used to bypass the addition of these TLVs.