fix(optee): Document CFLAGS for SDK toolchain

While building OP-TEE with the SDK's Yocto-generated toolchain, the
build fails with 'cannot find libgcc.a' linker error. This occurs
because the toolchain requires explicit sysroot configuration.

OP-TEE's mk/gcc.mk uses CFLAGS64 and CFLAGS32 while querying gcc for
library locations ensuring the compiler returns full paths. Add
documentation instructing users to export CFLAGS64 and CFLAGS32 with
--sysroot flags before building OP-TEE.

Signed-off-by: Shiva Tripathi <[email protected]>

Author: shiva-ti

source/linux/Foundational_Components_OPTEE.rst

@@ -38,6 +38,17 @@ Setting up the toolchain paths
    :start-after: .. start_include_yocto_toolchain_host_setup
    :end-before: .. end_include_yocto_toolchain_host_setup
 
+Additional Setup for OP-TEE
+^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+OP-TEE requires additional compiler flags to locate libraries correctly:
+
+.. code-block:: console
+
+   host# export CFLAGS64="--sysroot=${SYSROOT_64}"
+   host# export SYSROOT_32="${SDK_INSTALL_DIR}/k3r5-devkit/sysroots/armv7at2hf-vfp-oe-eabi"
+   host# export CFLAGS32="--sysroot=${SYSROOT_32}"
+
 Building OP-TEE OS
 ------------------