Add CFLAGS to makefile compile

From 8a5783e72029c1ecd26fb99130b48a470e13a8da Mon Sep 17 00:00:00 2001
From: Geoff Levand <geoff@infradead.org>
Date: Wed, 13 Nov 2013 12:26:01 -0800

Allow user to pass compiler flags on the make command line as
in 'make CFLAGS="-g"'.

Signed-off-by: Geoff Levand <geoff@infradead.org>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>