[bzimage] Align initrd images to page boundary

Some versions of Linux apparently complain if initrds are not aligned
to a page boundary.  Fix by changing INITRD_ALIGN from 4 bytes to 4096
bytes.

The amount of padding at the end of each initrd will now often be
sufficient to allow the cpio header to be prepended without crossing
an alignment boundary.  The final location of the initrd may therefore
end up being slightly higher than the post-shuffle location.
bzimage_load_initrd() must therefore now copy the initrd body prior to
copying the cpio header, otherwise the start of the initrd body may be
overwritten by the cpio header.  (Note that the guarantee that an
initrd will never need to overwrite an initrd at a higher location
still holds, since the overall length of each initrd cannot decrease
as a result of adding a cpio header.)

Reported-by: Dave Hansen <dave@sr71.net>
Signed-off-by: Michael Brown <mcb30@ipxe.org>