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 -durN dtc-e4b497f367a3b2ae99cc52089a14a221b13a76ef.orig/Makefile dtc-e4b497f367a3b2ae99cc52089a14a221b13a76ef/Makefile
--- dtc-e4b497f367a3b2ae99cc52089a14a221b13a76ef.orig/Makefile	2012-10-22 22:02:47.541240846 +0200
+++ dtc-e4b497f367a3b2ae99cc52089a14a221b13a76ef/Makefile	2012-10-22 22:03:21.151047833 +0200
@@ -18,7 +18,8 @@
 CPPFLAGS = -I libfdt -I .
 WARNINGS = -Werror -Wall -Wpointer-arith -Wcast-qual -Wnested-externs \
 	-Wstrict-prototypes -Wmissing-prototypes -Wredundant-decls
-CFLAGS = -g -Os -fPIC -Werror $(WARNINGS)
+CFLAGS ?= -g -Os
+CFLAGS += -fPIC $(WARNINGS)
 
 BISON = bison
 LEX = flex