cmake: add PLATFORM_VARIANT as a parameter in `Makefile.cmake`

This patch adds PLATFORM_VARIANT as an input parameter in
`Makefile.cmake` wrapper in order to improve how different
variants are build at the same time and standarize the way to
define multiple variants definition.

Signed-off-by: Leandro Belli <leandro.belli@arm.com>
Change-Id: I8304dea61b4aa62da65b028f43374664c4b9778b