doc: Add build instructions for OP-TEE backed EFI variables

Since that invlolves external projects and not only U-Boot, add guidance
for supported platforms

Signed-off-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
1 file changed