build: add -Wunused-const-variable=2 to W=2

TF-A is quite strict with warnings and redundant code. This flag
furthers this so it would be useful to have it. Add it to W=2 as it sets
off a few platforms which require a somewhat involved fix.

Signed-off-by: Boyan Karatotev <boyan.karatotev@arm.com>
Change-Id: Id52b3d477b4ada7dd69a36101ab22c575ab4ef19