Build: Search base config for tf-m-extras version

Search trusted-firmware-m config/config_base.cmake as that is now where
the variable is defined.

Change-Id: I9a8d4c8bc834a364ea1a4a8fc3f6ec7555c517d9
Signed-off-by: Jackson Cooper-Driver <jackson.cooper-driver@arm.com>
1 file changed