Make: Support CFLAGS as both env var and make argument

We usually pass arguments to make through environment variables:
CFLAGS="-std=c11" make quickcheck

It currently does not work to pass it as a make argument:
make quickcheck CFLAGS="-std=c11"

This commit fixes that.

Resolves #1315

Signed-off-by: Matthias J. Kannwischer <matthias@kannwischer.eu>

Author: mkannwischer

test/mk/config.mk

@@ -30,7 +30,8 @@ CC_AR  := $(CROSS_PREFIX)$(CC_AR)
 #################
 # Common config #
 #################
-CFLAGS := \
+
+override CFLAGS := \
 	-Wall \
 	-Wextra \
 	-Werror=unused-result \