Cmake: Add build error for deprecated platforms

Add an error message in the build system to inform the
user that the chosen platform won't be built cause it's in
the list of deprecated platforms.

Signed-off-by: Nicola Mazzucato <nicola.mazzucato@arm.com>
Change-Id: I7b8d64812fdb8eed941391c68e195b7ab412ce7e