boot/arm-trusted-firmware: don't force ENABLE_STACK_PROTECTOR

Commit 5f432df7 ("boot/arm-trusted-firmware: change
ENABLE_STACK_PROTECTOR value when disabled") set
ENABLE_STACK_PROTECTOR=0 when disabled. But since we pass this value as
MAKE_OPT, the internal ATF logic that sets ENABLE_STACK_PROTECTOR again
based on its initial value breaks. This leads to build failure:

make[1]: *** [/builds/buildroot.org/buildroot/output/build/arm-trusted-firmware-v2.4/build/a80x0_mcbin/release/libc/assert.o] Error 1
aarch64-buildroot-linux-uclibc-gcc.br_real: error: unrecognized command-line option ‘-fstack-protector-0’; did you mean ‘-fstack-protector’?

Move ENABLE_STACK_PROTECTOR to make environment instead to allow make to
change its value.

Fixes:
https://gitlab.com/buildroot.org/buildroot/-/jobs/1497663294



Cc: Dick Olsson <hi@senzilla.io>
Cc: Sergey Matyukevich <geomatsi@gmail.com>
Cc: Thomas Petazzoni <thomas.petazzoni@bootlin.com
Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>