ARM: rmobile: Contain CONFIG_ARCH_RMOBILE_BOARD_STRING

Pull the symbol from the boards and zap struct rmobile_sysinfo as they
are rather useless. The entire purpose of that whole machinery was to
print board name in the CONFIG_ARCH_RMOBILE_BOARD_STRING. Do that in a
far simpler and more contained manner.

Signed-off-by: Marek Vasut <marek.vasut+renesas@gmail.com>
Cc: Nobuhiro Iwamatsu <iwamatsu@nigauri.org>