bootutil: Allow bypassing ASN.1 encoding for ED25519 key import

The commit adds MCUBOOT_KEY_IMPORT_BYPASS_ASN configuration option
that allows bypassing ASN.1 decoding of ED25519 public key, compiled
into MCUboot.
When the option is enabled the key will be accessed directly
and ASN.1 processing is not compiled in, resulting in smaller
footprint of MCUboot, at a cost of reduced detection of invalid
key, i.e. public key designated for different method than
compiled in.

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