fdt: Ensure that libfdt_env.h comes from U-Boot

When building host utilities, we include libfdt.h from the host, not from
U-Boot. This in turn brings in libfdt_env.h from the host, which can mess
up the types and cause a build failure, depending on the host environment.
To fix this, force inclusion of U-Boot's libfdt_env.h so that the types
are correct.

Another way to fix this is to use -nostdinc and -idirafter to ensure that
system includes are included after U-Boot ones. Unfortunately this means
that U-Boot's errno.h gets included instead of the system one. This in
turn requires a hack to errno.h to redirect things, so all in all the
solution in this patch is probably cleaner.

Signed-off-by: Simon Glass <sjg@chromium.org>