travis-ci: Re-work i.MX6 jobs, clarify Freescale and AArch64

- The catch-all i.MX6 job has been exceeding the time limit again so
  split this up further.  We now have an i.MX6 job and an
  everything-else job.
- The logic we use to say "Freescale and AArch64" can be more clearly
  expressed with '&' rather than excluding various other things, so
  clear that up.

Signed-off-by: Tom Rini <trini@konsulko.com>