kconfig: add CONFIG_CC_OPTIMIZE_FOR_SIZE

Copy the Kconfig option from "init/Kconfig" of Linux v3.16 tag
and adjust the help document.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
diff --git a/Makefile b/Makefile
index 0fea5c2..9646859 100644
--- a/Makefile
+++ b/Makefile
@@ -529,7 +529,11 @@
 include/config/auto.conf: ;
 endif # $(dot-config)
 
-KBUILD_CFLAGS += -Os #-fomit-frame-pointer
+ifdef CONFIG_CC_OPTIMIZE_FOR_SIZE
+KBUILD_CFLAGS	+= -Os
+else
+KBUILD_CFLAGS	+= -O2
+endif
 
 ifdef BUILD_TAG
 KBUILD_CFLAGS += -DBUILD_TAG='"$(BUILD_TAG)"'