boot/arm-trusted-firmware: add SSP option

Buildroot sets appropriate ENABLE_STACK_PROTECTOR build flag value based
on the toolchain global BR2_SSP_* options, and all packages are built
with that setting.

However it might not be always convenient to automatically infer TF-A
stack protection from the toolchain features. For instance, secure
memory constraints may become an issue and all the extra TF-A features
need to be tuned or disabled in order to shrink TF-A firmware image.

Besides, for any value other than "none", TF-A platform specific hook
'plat_get_stack_protector_canary' must be implemented. However this hook
is not implemented by all the platforms supported by TF-A. For instance,
Allwinner currently does not provide such a hook.

Add an new option that a user can toggle to enable or disable SSP in
their ATF build. If enabled, the SSP level is automatically inherited
from the global setting.

Signed-off-by: Sergey Matyukevich <geomatsi@gmail.com>
[yann.morin.1998@free.fr: simplify logic with a single boolean]
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>