Build: Add error when bad options are given

When either of the two required input paramters (CMAKE_TOOLCHAIN_FILE
and TFM_PLATFORM) are given, validate that they are sane values and
output a reasonable error message if they are not.

Change-Id: Ie3a93ffed331e5ed66483f0b22c7875db469700c
Signed-off-by: Raef Coles <raef.coles@arm.com>
diff --git a/toolchain_GNUARM.cmake b/toolchain_GNUARM.cmake
index bb1728c..d1b4e1f 100644
--- a/toolchain_GNUARM.cmake
+++ b/toolchain_GNUARM.cmake
@@ -5,6 +5,8 @@
 #
 #-------------------------------------------------------------------------------
 
+set(TFM_CMAKE_TOOLCHAIN_FILE_LOADED YES)
+
 set(CMAKE_SYSTEM_NAME Generic)
 set(CMAKE_SYSTEM_PROCESSOR arm)