Fix a variable expansion in Arm platform makefiles

The top level makefile defines the PLAT variable, not PLATFORM.
This mistake was causing an empty variable expansion and showing an
incomplete error message.

Change-Id: I5da1275c73c61a7c1823643a76300f255841719d
Signed-off-by: Sandrine Bailleux <sandrine.bailleux@arm.com>