sparc: Use nicer prompt for board select menu

The Board names in the board select menu were simply taken from the
board directory name by a conversion tool when switching to Kconfig.

Let's use more descriptive prompts taken from
include/configs/gr*.h and board/gaisler/*/config.mk.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
Cc: Daniel Hellstrom <daniel@gaisler.com>