libfdt: Trivial cleanup for CHECK_HEADER)

Currently the CHECK_HEADER() macro is defined local to fdt_ro.c.
However, there are a handful of functions (fdt_move, rw_check_header,
fdt_open_into) from other files which could also use it (currently
they open-code something more-or-less identical).  Therefore, this
patch moves CHECK_HEADER() to libfdt_internal.h and uses it in those
places.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
diff --git a/libfdt/fdt.c b/libfdt/fdt.c
index c61fb53..bd91712 100644
--- a/libfdt/fdt.c
+++ b/libfdt/fdt.c
@@ -184,10 +184,7 @@
 
 int fdt_move(const void *fdt, void *buf, int bufsize)
 {
-	int err = fdt_check_header(fdt);
-
-	if (err)
-		return err;
+	CHECK_HEADER(fdt);
 
 	if (fdt_totalsize(fdt) > bufsize)
 		return -FDT_ERR_NOSPACE;