boot/optee-os: new package

boot/optee-os: new package

OP-TEE OS is maintained by the OP-TEE project. It provides an
open source solution for development and integration of secure
services for Armv7-A and Armv8-A CPU based platforms supporting
the TrustZone technology. This technology enables CPUs to
concurrently host a secure world as the OP-TEE OS and a non-secure
world as a Linux based OS.

The OP-TEE project maintains other packages to leverage OP-TEE on
Linux kernel based OSes. An OP-TEE interface driver is available
in the Linux kernel since 4.12 upon CONFIG_OPTEE.

This change references in Buildroot the today's latest OP-TEE
revision release tagged 3.4.0.

https://www.op-tee.org/
https://github.com/OP-TEE/optee_os

Signed-off-by: Etienne Carriere <etienne.carriere@linaro.org>
[Thomas:
 - pass CFG_ARM32_core=y when building for ARMv7, otherwise the build
   fails
 - add a check that verifies that BR2_TARGET_OPTEE_OS_PLATFORM is not
   empty
 - minor formatting fixes/adjustements.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>