Build: exclude -c flag from TF_CFLAGS

The -c flag should not be included in the global variable TF_CFLAGS;
it should be specified in the build rule only when its target is a
*.o file.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
diff --git a/Makefile b/Makefile
index e8716e5..bb46afc 100644
--- a/Makefile
+++ b/Makefile
@@ -149,7 +149,7 @@
 				${DEFINES} ${INCLUDES}
 TF_CFLAGS		+= 	-nostdinc -ffreestanding -Wall			\
 				-Werror -Wmissing-include-dirs			\
-				-std=c99 -c -Os					\
+				-std=c99 -Os					\
 				$(TF_CFLAGS_$(ARCH))				\
 				${DEFINES} ${INCLUDES}
 TF_CFLAGS		+=	-ffunction-sections -fdata-sections