libfdt: migrate include/libfdt_env.h to a wrapper

libfdt_env.h is supposed to provide system-dependent defines.

scripts/dtc/libfdt/libfdt_env.h from upstream DTC is suitable
for user-space, so we should use this for USE_HOSTCC case.

For compiling U-Boot, we need to override such system-dependent
defines, so use <linux/libfdt_env.h> imported from Linux.

<libfdt.h> selects a proper one.  Maybe, we should split header
inclusion completely, but I do not want too many patches at one.

I can rip off the include/libfdt_env.h from HOST_EXTRACFLAGS.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Reviewed-by: Simon Glass <sjg@chromium.org>