sh: allow romImage data between head.S and the zero page

Extend the romImage code to allow putting data between
the head.S file and the empty_zero_page. Needed in the
case of more advanced loader code in a separate C file.

Signed-off-by: Magnus Damm <damm@opensource.se>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>