ARM: rmobile: Fix CONFIG_RMOBILE_BOARD_STRING

Rename CONFIG_RMOBILE_BOARD_STRING to CONFIG_ARCH_RMOBILE_BOARD_STRING
to make things consistent, no functional change.

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