[Buildroot] [BUG] boot/arm-trusted-firmware: Stack protection setting doesn't work

[Daniil Stas]

The bug was introduced in commit ccac9a5bbb
("boot/arm-trusted-firmware: don't force ENABLE_STACK_PROTECTOR").

In this commit the ENABLE_STACK_PROTECTOR option was moved from make
arguments to the environment. But this value gets overwritten in
make_helpers/defaults.mk (and platform specific makefiles) before it
gets handled in the lib/stack_protector/stack_protector.mk.
So setting it via environment doesn't work.

You can check it, for example, if you compile TF-A for stm32mp157a_dk1
platform, and then check if bl32/stack_protector.o file is
built.
It gets built only with commit ccac9a5bbb being reverted.