travis: Combine kernel builds.

travis: Combine kernel builds.

Single kernel build job takes ~3 minutes in average. Most of
this time takes VM spawning and initial configuration.
Combining these 24 jobs in 4 allows us to better utilize workers
and not waste time on spawning VMs.

Before:

    24 jobs * 3 minutes = 72 minutes
    72 minutes / 5 workers = 14.4 minutes / worker

After:

    4 jobs * 10 minutes = 40 minutes
    40 minutes / 4 workers = 10 minutes / worker
    + 1 free worker that able to run other jobs at the same time.

Signed-off-by: Ilya Maximets <i.maximets@samsung.com>
Acked-by: Aaron Conole <aconole@redhat.com>