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>
diff --git a/arch/sandbox/cpu/Makefile b/arch/sandbox/cpu/Makefile
index 58c2537..b564294 100644
--- a/arch/sandbox/cpu/Makefile
+++ b/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)))