sh: Add handling of CONFIG_SYS_NO_FLASH for board.c

Some board of SH does not have flash memoy.
This revises it to initialize Flash when CONFIG_SYS_NO_FLASH is not
defined.

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