hikey: Add development TBB support

hikey: Add development TBB support

This patch adds experimental support for TRUSTED_BOARD_BOOT to the
Hikey. This is adapted from the RPi3 and QEMU implementations.

Since the Hikey starts from BL2 the TRUSTED_BOARD_BOOT ROT begins there
too. When TRUSTED_BOARD_BOOT is defined, the BL1 build is skipped.

See the following example:

make \
 PLAT=hikey \
 BL33=u-boot.bin \
 SCP_BL2=mcuimage.bin \
 TRUSTED_BOARD_BOOT=1 \
 MBEDTLS_DIR=../../mbedtls \
 GENERATE_COT=1 \
 all fip

Signed-off-by: Teddy Reed <teddy.reed@gmail.com>