build(cmake): add platform variant support

Some platforms offer multiple variants, targeting different but similar
physical boards. For example, the Total Compute platform currently
offers both TC1 and TC2. We'll be using this section to dictate
information that needs to be interpreted before loading the toolchain.

Change-Id: I9fe7450ab42c398a7ca42e0b2303df199fac8567
Signed-off-by: Chris Kay <chris.kay@arm.com>