sh: Add support any page size and empty_zero_page to SH Linux uImage

Old U-Boot supported 4KB page size only. If this version, Linux
kernel can not get command line from U-Boot.
SH Linux kernel can change page size and empty_zero_page.
This patch support this function and fix promlem.

Signed-off-by: Nobuhiro Iwamatsu <iwamatsu.nobuhiro@renesas.com>
Signed-off-by: Nobuhiro Iwamatsu <iwamatsu@nigauri.org>