arch: arm: core: tz: Linking of secure entry functions into NS firmware

Add Kconfig options:
 - ARM_FIRMWARE_USES_SECURE_ENTRY_FUNCS
 - ARM_ENTRY_VENEERS_LIB_NAME

Use these to link the veneers lib into the Non-Secure Firmware when
needed.

Also, make the path passed to the linker absolute to make it work with
makefiles.

Signed-off-by: Øyvind Rønningstad <oyvind.ronningstad@nordicsemi.no>