kbuild: move extra gcc checks to scripts/Makefile.extrawarn

This commit was imported from Linux Kernel:
commit a86fe353 written by me.

W=... provides extra gcc checks.

Having such code in scripts/Makefile.build results in the same flags
being added to KBUILD_CFLAGS multiple times becuase
scripts/Makefile.build is invoked every time Kbuild descends into
the subdirectories.

Since the top Makefile is already too cluttered, this commit moves
all of extra gcc check stuff to a new file scripts/Makefile.extrawarn,
which is included from the top Makefile.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>

Makefile

@@ -109,10 +109,6 @@ ifeq ("$(origin O)", "command line")
   KBUILD_OUTPUT := $(O)
 endif
 
-ifeq ("$(origin W)", "command line")
-  export KBUILD_ENABLE_EXTRA_GCC_CHECKS := $(W)
-endif
-
 # That's our default target when none is given on the command line
 PHONY := _all
 _all:
@@ -569,6 +565,8 @@ endif
 
 export CONFIG_SYS_TEXT_BASE
 
+include $(srctree)/scripts/Makefile.extrawarn
+
 # Add user supplied CPPFLAGS, AFLAGS and CFLAGS as the last assignments
 KBUILD_CPPFLAGS += $(KCPPFLAGS)
 KBUILD_AFLAGS += $(KAFLAGS)