Makefile: append the CFLAGS to existing ones

Allow the user to pass custom CFLAGS (eg. optimisation flags).

Do not use EXTRA_CFLAGS, append to existing CFLAGS with +=  (Arnout)

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Arnout Vandecappelle <arnout@mind.be>

---
Patch not sent upstream.

Although not specific to buildroot, I am not sure this is the best
way to handle user-supplied CFLAGS.

diff --git a/Makefile b/Makefile
index 962f94eba661..bf6b317158cf 100644
--- a/Makefile
+++ b/Makefile
@@ -16,9 +16,10 @@ LOCAL_VERSION =
 CONFIG_LOCALVERSION =
 
 CPPFLAGS = -I libfdt -I .
-WARNINGS = -Werror -Wall -Wpointer-arith -Wcast-qual -Wnested-externs \
+WARNINGS = -Wall -Wpointer-arith -Wcast-qual -Wnested-externs \
 	-Wstrict-prototypes -Wmissing-prototypes -Wredundant-decls -Wshadow
-CFLAGS = -g -Os -fPIC -Werror $(WARNINGS)
+CFLAGS ?= -g -Os
+CFLAGS += -fPIC $(WARNINGS)
 
 BISON = bison
 LEX = flex