nios2: move generic config to boards.cfg

I can't build test this, but just looking at the config files written
and it seems OK ...

Tested-by: Thomas Chou <thomas@wytron.com.tw>
Signed-off-by: Mike Frysinger <vapier@gentoo.org>
Signed-off-by: Scott McNutt <smcnutt@psyent.com>