libfdt: move headers to <linux/libfdt.h> and <linux/libfdt_env.h>

Thomas reported U-Boot failed to build host tools if libfdt-devel
package is installed because tools include libfdt headers from
/usr/include/ instead of using internal ones.

This commit moves the header code:
  include/libfdt.h         -> include/linux/libfdt.h
  include/libfdt_env.h     -> include/linux/libfdt_env.h

and replaces include directives:
  #include <libfdt.h>      -> #include <linux/libfdt.h>
  #include <libfdt_env.h>  -> #include <linux/libfdt_env.h>

Reported-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>

lib/libfdt/Makefile

@@ -23,3 +23,5 @@ obj-y += fdt_ro.o
 
 # U-Boot own file
 obj-y += fdt_region.o
+
+ccflags-y := -I$(srctree)/scripts/dtc/libfdt

lib/libfdt/fdt_region.c

@@ -5,11 +5,11 @@
  * SPDX-License-Identifier:	GPL-2.0+ BSD-2-Clause
  */
 
-#include <libfdt_env.h>
+#include <linux/libfdt_env.h>
 
 #ifndef USE_HOSTCC
 #include <fdt.h>
-#include <libfdt.h>
+#include <linux/libfdt.h>
 #else
 #include "fdt_host.h"
 #endif

lib/libfdt/fdt_ro.c

@@ -3,11 +3,11 @@
  * Copyright (C) 2006 David Gibson, IBM Corporation.
  * SPDX-License-Identifier:	GPL-2.0+ BSD-2-Clause
  */
-#include <libfdt_env.h>
+#include <linux/libfdt_env.h>
 
 #ifndef USE_HOSTCC
 #include <fdt.h>
-#include <libfdt.h>
+#include <linux/libfdt.h>
 #else
 #include "fdt_host.h"
 #endif