[review] arm32: paging of TEE Core optionally enabled

plat-stm:
* Displays an error message if compiled with CFG_WITH_PAGER=y

Signed-off-by: Jens Wiklander <jens.wiklander@linaro.org>
[review] arm32: paging of TEE Core optionally enabled

* Adds tee_ prefix to all pager_ prefix
* Removes unused tee_pager_unpg.c

Signed-off-by: Jens Wiklander <jens.wiklander@linaro.org>
travis: Add CFG_WITH_PAGER=1 to qemu_virt

Adds CFG_WITH_PAGER=1 compilation for PLATFORM=vexpress-qemu_virt

Signed-off-by: Jens Wiklander <jens.wiklander@linaro.org>