travis-ci: compile with buildman when running test/py

Use buildman to compile any U-Boot binary tested by test/py. This
re-uses all the work done elsewhere to make buildman work within
Travis-CI, in particular related to toolchain downloading and buildman
config file creation.

Signed-off-by: Stephen Warren <swarren@nvidia.com>
Reviewed-by: Heiko Schocher <hs@denx.de>