data_smart: Allow numeric characters in overrides

We're seeing problems due to the way x86-64 is handled (or not handled)
as an override. Relax the containts on overrides from being lowercase
to being lowercase or numeric. This fixes problem where MACHINE=qemux86
would work but MACHINE=qemux86-64 would fail the same tests.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>