libfdt: migrate libfdt.h to a wrapper + U-Boot own code

There is tons of code duplication between lib/libfdt/libfdt.h and
scripts/dtc/libfdt/libfdt.h.  Evacuate the U-Boot own code to
include/libfdt.h and remove lib/libfdt/libfdt.h.

For host tools, <libfdt.h> should include scripts/dtc/libfdt/libfdt.h,
which is already suitable for user-space.

For compiling U-Boot, <linux/libfdt.h> should be included because we
need a different libfdt_env.h .

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>