uboot-phytec

travis-ci: Emulate 'make tests'

The 'tests' target will run sandbox, sandbox_spl and sandbox_flattree in
test.py and in the case of sandbox_spl ensure that we just run the
specific tests for that build.  Update our matrix to perform similar
test.py runs.

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