imgtool: Add clear image generation with encryption capability

        return cipherkey, ciphermac, pubk

    def create(self, key, public_key_format, enckey, dependencies=None,

               sw_type=None, custom_tlvs=None, encrypt_keylen=128):

               sw_type=None, custom_tlvs=None, encrypt_keylen=128, clear=False):

        self.enckey = enckey

        # Calculate the hash of the public key

                else:

                    tlv.add('ENCX25519', enctlv)

            if not clear:

                nonce = bytes([0] * 16)

                cipher = Cipher(algorithms.AES(plainkey), modes.CTR(nonce),

                                backend=default_backend())

              type=click.Choice(['128','256']),

              help='When encrypting the image using AES, select a 128 bit or '

                   '256 bit key len.')

@click.option('-c', '--clear', required=False, is_flag=True, default=False,

              help='Output a non-encrypted image with encryption capabilities,'

                   'so it can be installed in the primary slot, and encrypted '

                   'when swapped to the secondary.')

@click.option('-e', '--endian', type=click.Choice(['little', 'big']),

              default='little', help="Select little or big endian")

@click.option('--overwrite-only', default=False, is_flag=True,

         pad_header, slot_size, pad, confirm, max_sectors, overwrite_only,

         endian, encrypt_keylen, encrypt, infile, outfile, dependencies,

         load_addr, hex_addr, erased_val, save_enctlv, security_counter,

         boot_record, custom_tlv, rom_fixed, max_align):

         boot_record, custom_tlv, rom_fixed, max_align, clear):

    if confirm:

        # Confirmed but non-padded images don't make much sense, because

            custom_tlvs[tag] = value.encode('utf-8')

    img.create(key, public_key_format, enckey, dependencies, boot_record,

               custom_tlvs, int(encrypt_keylen))

               custom_tlvs, int(encrypt_keylen), clear)

    img.save(outfile, hex_addr)