Following build flags have been added to support optional firmware
encryption:

- FW_ENC_STATUS: Top level firmware's encryption numeric flag, values:
    0: Encryption is done with Secret Symmetric Key (SSK) which is
       common for a class of devices.
    1: Encryption is done with Binding Secret Symmetric Key (BSSK) which
       is unique per device.

- ENC_KEY: A 32-byte (256-bit) symmetric key in hex string format. It
    could be SSK or BSSK depending on FW_ENC_STATUS flag.

- ENC_NONCE: A 12-byte (96-bit) encryption nonce or Initialization Vector
    (IV) in hex string format.

- ENCRYPT_BL31: Binary flag to enable encryption of BL31 firmware.

- ENCRYPT_BL32: Binary flag to enable encryption of Secure BL32 payload.

Similar flags can be added to encrypt other firmwares as well depending
on use-cases.

Signed-off-by: Sumit Garg <sumit.garg@linaro.org>
Change-Id: I94374d6830ad5908df557f63823e58383d8ad670