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 <[email protected]>
Change-Id: Id52b3d477b4ada7dd69a36101ab22c575ab4ef19

Author: loosper-arm

Makefile

@@ -374,6 +374,8 @@ WARNING2 += -Wmissing-prototypes
 WARNING2 += -Wmissing-format-attribute
 # TF-A aims to comply with this eventually. Effort too large at present
 WARNING2 += -Wundef
+# currently very involved and many platforms set this off
+WARNING2 += -Wunused-const-variable=2
 
 # Level 3 - very pedantic, frequently ignored
 WARNING3 := -Wbad-function-cast