libfdt: Trivial cleanup for CHECK_HEADER)

[Jon Loeliger]

So, like, the other day David Gibson mumbled:
> Currently the CHECK_HEADER() macro is defined local to fdt_ro.c.
> However, there are a handful of functions (fdt_move, rw_check_header,
> fdt_open_into) from other files which could also use it (currently
> they open-code something more-or-less identical).  Therefore, this
> patch moves CHECK_HEADER() to libfdt_internal.h and uses it in those
> places.
> 
> Signed-off-by: David Gibson <david at gibson.dropbear.id.au>

Applied.

jdl