test/py: Allow using buildman to build U-Boot

It is a pain to have to set the CROSS_COMPILE environment variable when
using test.py's --build option. It is possible to get this using the -A
option from buildman. But it seems better to just use buildman to do the
build when it is available.

However using buildman adds a new dependency to the test system which we
want to avoid. So leave the default as is and add a flag to make it use
buildman.

Note that most of these changes relate to test.py and the parts of the
travis/gitlab/azure scripts which relate to running test and building a
suitable U-Boot to run the tests on.

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