x86: Enable DIAG LED on Geos

Based on a patch from Chris Blake <chrisrblake93@gmail.com>, except let's
do it by using the LED configuration instead of hard-coding it for each
board type. And try using /bin/board_detect to do the default behaviour,
on the first boot where the config hasn't yet been generated.

Signed-off-by: David Woodhouse <dwmw2@infradead.org>