travis: Add sandbox/clang-7 support

To make testing with clang support easier, add sandbox/clang-7
combination to our testing matrix.  To facilitate this, switch to using
the "sources" method that the travis.yml file supports to list
additional repositories and add the official one for llvm-7.  Due to
buildman not supporting using clang at this time add logic to manually
build a single sandbox configuration in the expected output directory so
that we can still invoke all of our tests.

Cc: Simon Glass <sjg@chromium.org>
Signed-off-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Simon Glass <sjg@chromium.org>

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