boot/arm-trusted-firmware: add support for using OP-TEE as BL32

boot/arm-trusted-firmware: add support for using OP-TEE as BL32

This change allows one to build trusted firmware (TF-A) with OP-TEE as
BL32 secure payload.

When BR2_TARGET_ARM_TRUSTED_FIRMWARE_BL32_DEFAULT is enabled TF-A
builds a BL32 stage according the TF-A configuration directive.  If
these specify no BL32 stage then TF-A will build without BL32
support. This is the default configuration and reflects TF-A legacy
integration in BR.

When BR2_TARGET_ARM_TRUSTED_FIRMWARE_BL32_OPTEE is enabled TF-A builds
with support for the OP-TEE OS as BL32.

Signed-off-by: Etienne Carriere <etienne.carriere@linaro.org>
[Thomas: rename config options, simplify option prompts, and rework
option description.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>