build: add Wextra compile option

This patch add -Wextra compile option to RMM build. The
warning for implicit fallthrough is disabled as this is a
pattern used in the code. Couple of warnings triggered by
-Wextra are also fixed as part of the patch.


Change-Id: I7c0b815e5fadd3d45d4c72508df64d77f916d6d3
Signed-off-by: Soby Mathew <soby.mathew@arm.com>
9 files changed