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/lib/libfdt/Makefile b/lib/libfdt/Makefile
index 4e3e12f..edd8e64 100644
--- a/lib/libfdt/Makefile
+++ b/lib/libfdt/Makefile
@@ -23,3 +23,5 @@
 
 # U-Boot own file
 obj-y += fdt_region.o
+
+ccflags-y := -I$(srctree)/scripts/dtc/libfdt