libfdt: Trivial cleanup for CHECK_HEADER)

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@gibson.dropbear.id.au>