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>
diff --git a/board/freescale/ls1088a/ls1088a.c b/board/freescale/ls1088a/ls1088a.c
index 0769e90..56e454f 100644
--- a/board/freescale/ls1088a/ls1088a.c
+++ b/board/freescale/ls1088a/ls1088a.c
@@ -13,7 +13,7 @@
 #include <fsl_sec.h>
 #include <asm/io.h>
 #include <fdt_support.h>
-#include <libfdt.h>
+#include <linux/libfdt.h>
 #include <fsl-mc/fsl_mc.h>
 #include <environment.h>
 #include <asm/arch-fsl-layerscape/soc.h>