sandbox: Use system headers first for sandbox's os.c

This file must be compiled with system headers, even if U-Boot has headers
of the same name. The existing solution for this is good enough for libfdt,
but fails when we have headers like stdint.h in U-Boot.

Use -idirafter instead of -I, and remove the -nostdinc and other things
that we don't want for this file. The best way to do this is to keep a
copy of the original flags, rather than trying to filter them later.

Signed-off-by: Simon Glass <sjg@chromium.org>

arch/sandbox/cpu/Makefile

@@ -10,5 +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 := $(filter-out -nostdinc,$(ALL_CFLAGS))
-$(obj).depend.os: CPPFLAGS := $(filter-out -nostdinc,$(CPPFLAGS))
+$(obj)os.o: ALL_CFLAGS := $(BASE_CPPFLAGS) \
+	$(patsubst %, -idirafter %, $(BASE_INCLUDE_DIRS))
+$(obj).depend.os: CPPFLAGS := $(BASE_CPPFLAGS) \
+	$(patsubst %, -idirafter %, $(BASE_INCLUDE_DIRS))