Build: Add warning about absolute toolchain paths

Note that CMAKE_TOOLCHAIN_FILE no longer accepts relative paths.
Implemented as an error as a relative path will also cause an error.

Change-Id: Ifccaef5b7738c4867031e2d1d5cd37fbf24f7cf6
Signed-off-by: Raef Coles <raef.coles@arm.com>
1 file changed