.travis.yml: collapse the test matrix

Remove the concept of TARGETS and build the complete target list for
each config combination. Now the matrix is just based on CONFIG stanzas
and we use the additional stuff for:

  - things that only work on one compiler (sparse, gcov, gprof)
  - combos where "make check" fails

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