build_helper_configs.py: Select target tfm_ns when BL2=OFF

tfm_app_binaries are built only when BL2=ON.
Build target tfm_ns instead when BL2=OFF.
Pass PLATFORM_DEFAULT_IMAGE_SIGNING to NS build to control image signing
together with BL2.

It is a temporal workaround to mitigate the difference between
toolchains.

Change-Id: I221d16fd36ddbd5a655bb0132c94ebefcd344b8c
Signed-off-by: David Hu <david.hu@arm.com>
2 files changed