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/cmd/spl.c b/cmd/spl.c
index 3b8992a..5939897 100644
--- a/cmd/spl.c
+++ b/cmd/spl.c
@@ -8,7 +8,7 @@
 #include <common.h>
 #include <command.h>
 #include <cmd_spl.h>
-#include <libfdt.h>
+#include <linux/libfdt.h>
 
 DECLARE_GLOBAL_DATA_PTR;