sandbox: Use system headers first for sandbox's os.c in a different way

Commit cbe5cdfcd changed config.mk and arch/sandbox/cpu/Makefile
to use -idirafter instead of -I and remove -nostdinc.

But
 * Sandbox-specific code dirties config.mk
 * os.c is compiled without such compiler flags as:
      -Wall -Wstrict-prototypes -Wno-format-security
      -fno-builtin -ffreestanding -fno-stack-protector
      -fstack-usage -Wno-format-nonliteral

This commit use -idirafter and remove the -nostdinc
differently and more simply.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
Acked-by: Simon Glass <sjg@chromium.org>

arch/sandbox/cpu/Makefile

@@ -10,7 +10,7 @@
 obj-y	:= cpu.o os.o start.o state.o
 
 # os.c is build in the system environment, so needs standard includes
-$(obj)os.o: ALL_CFLAGS := $(BASE_CPPFLAGS) \
-	$(patsubst %, -idirafter %, $(BASE_INCLUDE_DIRS))
-$(obj).depend.os: CPPFLAGS := $(BASE_CPPFLAGS) \
-	$(patsubst %, -idirafter %, $(BASE_INCLUDE_DIRS))
+$(obj)os.o: CFLAGS := $(filter-out -nostdinc,\
+	$(patsubst -I%,-idirafter%,$(CFLAGS)))
+$(obj).depend.os: CPPFLAGS := $(filter-out -nostdinc,\
+	$(patsubst -I%,-idirafter%,$(CPPFLAGS)))

config.mk

@@ -250,16 +250,11 @@ Please undefined CONFIG_SYS_GENERIC_BOARD in your board config file)
 endif
 endif
 
-# Sandbox needs the base flags and includes, so keep them around
-BASE_CPPFLAGS := $(CPPFLAGS)
-
 ifneq ($(OBJTREE),$(SRCTREE))
-BASE_INCLUDE_DIRS := $(OBJTREE)/include
+CPPFLAGS += -I$(OBJTREE)/include
 endif
 
-BASE_INCLUDE_DIRS += $(TOPDIR)/include $(SRCTREE)/arch/$(ARCH)/include
-
-CPPFLAGS += $(patsubst %, -I%, $(BASE_INCLUDE_DIRS))
+CPPFLAGS += -I$(TOPDIR)/include -I$(SRCTREE)/arch/$(ARCH)/include
 CPPFLAGS += -fno-builtin -ffreestanding -nostdinc	\
 	-isystem $(gccincdir) -pipe $(PLATFORM_CPPFLAGS)