boot: Rename boot_enc_decrypt to boot_decrypt_key

All of boot_enc_ function follow the same pattern where
they take encryption context as the first parameter, and the
boot_enc_decrypt stands out here as it does not work around
the encryption context, but is rather single-part decryption
function only used for decrypting of the image encryption
key.

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