config: Fix SCRATCH_DEV_POOL handling

config: Fix SCRATCH_DEV_POOL handling

With changes introduced in 667308dd97bf41382d4ab299fa5b56c235cfeb27
it is no longer possible to use SCRATCH_DEV_POOL variable because of
error:

 common/config: Error: $SCRATCH_DEV should be unset when
 $SCRATCH_DEV_POOL is set

This was because get_next_config() would get called twice and hence it
would complain on the second run that SCRATCH_DEV is already set. Fix
it by making sure that we call get_next_config() only once if there
are no sections in the config file.

Also make sure that we export SCRATCH_DEV in the case we're deducing it
from SCRATCH_DEV_POOL.

Signed-off-by: Lukas Czerner <lczerner@redhat.com>
Reported-by: Filipe David Manana <fdmanana@gmail.com>
Tested-by: Filipe David Manana <fdmanana@gmail.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>