Pass --target and --sysroot to dtc as CPPFLAGS

This is necessary for the depfiles to be correct when cross-compiling.
diff --git a/Makefile b/Makefile
index 13dd708..0231504 100644
--- a/Makefile
+++ b/Makefile
@@ -529,7 +529,7 @@
 # LIBFDT_lib="": avoid breaking existing trees with objects requiring -fPIC
 DTC_MAKE_ARGS=-I$(SRC_PATH)/dtc VPATH=$(SRC_PATH)/dtc -C dtc V="$(V)" LIBFDT_lib=""
 DTC_CFLAGS=$(CFLAGS) $(QEMU_CFLAGS)
-DTC_CPPFLAGS=-I$(SRC_PATH)/dtc/libfdt
+DTC_CPPFLAGS=-I$(SRC_PATH)/dtc/libfdt $(filter --target% --sysroot%,$(QEMU_CFLAGS))
 
 .PHONY: dtc/all
 dtc/all: .git-submodule-status dtc/libfdt