libfdt: Make fdt_check_header() public

Changed _fdt_check_header() to fdt_check_header() and made it part of
the interface - it is a useful routine.

Also did some asthetics cleanup to the include files (headers).
diff --git a/libfdt/fdt_ro.c b/libfdt/fdt_ro.c
index ce01dc7..2711324 100644
--- a/libfdt/fdt_ro.c
+++ b/libfdt/fdt_ro.c
@@ -25,7 +25,7 @@
 
 #define CHECK_HEADER(fdt)	{ \
 	int err; \
-	if ((err = _fdt_check_header(fdt)) != 0) \
+	if ((err = fdt_check_header(fdt)) != 0) \
 		return err; \
 }
 
@@ -188,7 +188,7 @@
 	int offset, nextoffset;
 	int err;
 
-	if ((err = _fdt_check_header(fdt)) != 0)
+	if ((err = fdt_check_header(fdt)) != 0)
 		goto fail;
 
 	err = -FDT_ERR_BADOFFSET;