Enable user-overriable CFLAGS

Put required CFLAGS (ie pkg-config --cflags) in ALL_CFLAGS and append
user specified CFLAGS to that.

Use CFLAGS during link.

Signed-off-by: Brad Bishop <bradleyb@fuzziesquirrel.com>
diff --git a/op-flasher/Makefile b/op-flasher/Makefile
index 80b9ccb..6876ad0 100644
--- a/op-flasher/Makefile
+++ b/op-flasher/Makefile
@@ -4,11 +4,11 @@
 	    libflash.o \
 	    libffs.o \
 	    arm_io.o
-CFLAGS+=-Ipflash
+ALL_CFLAGS+=-Ipflash
 include ../gdbus.mk
 include ../rules.mk
 
 %.o: pflash/%.c
-	$(CC) -c -o $(CFLAGS) -fPIC -o $@ $<
+	$(CC) -c $(ALL_CFLAGS) -o $@ $<
 %.o: pflash/libflash/%.c
-	$(CC) -c -o $(CFLAGS) -fPIC -o $@ $<
+	$(CC) -c $(ALL_CFLAGS) -o $@ $<