Move platform-specific flags to their own files and use them in TA dev kit

Platform-specific flags that do not apply only to the TEE core are removed
from core/arch/$(ARCH)plat-$(PLATFORM)/conf.mk. They are moved to a new file:
mk/arch/$(ARCH)/plat-$(PLATFORM)/platform_flags.mk.

This file is used by ta/mk/ta_dev_kit.mk so that the Trusted Applications
are built with the same flags used when building the user-mode TEE code.

Signed-off-by: Jerome Forissier <jerome.forissier@linaro.org>