libfdt: Rename and publish _fdt_check_header()

It's potentially useful for users of libfdt to sanity check a device
tree (or, rather, a blob of data which may or may not be a device
tree) before processing it in more detail with libfdt.

This patch renames the libfdt internal function _fdt_check_header() to
fdt_check_header() and makes it a published function, so it can now be
used for this purpose.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
diff --git a/libfdt/fdt.c b/libfdt/fdt.c
index a29f1b0..2911b42 100644
--- a/libfdt/fdt.c
+++ b/libfdt/fdt.c
@@ -55,7 +55,7 @@
 
 #include "libfdt_internal.h"
 
-int _fdt_check_header(const void *fdt)
+int fdt_check_header(const void *fdt)
 {
 	if (fdt_magic(fdt) == FDT_MAGIC) {
 		/* Complete tree */
@@ -143,7 +143,7 @@
 
 int fdt_move(const void *fdt, void *buf, int bufsize)
 {
-	int err = _fdt_check_header(fdt);
+	int err = fdt_check_header(fdt);
 
 	if (err)
 		return err;