sh: disable initrd defaults in .empty_zero_page.

When using initramfs on systems that don't explicitly clear LOADER_TYPE,
unpack_to_rootfs() tramples tramples the range with the defaults taken
out of .empty_zero_page. This causes kernels with valid initramfs images
to bail out with crc or gzip magic mismatch errors after the second
unpack takes place on certain platform configurations.

Signed-off-by: Paul Mundt <lethal@linux-sh.org>