.travis.yml: reduce the test matrix a little

As we are now running "make check" on more of the matrix it is worth
making more of an effort to reduce the overall load on Travis. I've done
a few things:

  - Combining a number of the targets
  - Building one target for each ancillary build

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: David Gibson <david@gibson.dropbear.id.au>
Tested-by: David Gibson <david@gibson.dropbear.id.au>
