travis-ci: use buildman -P everywhere

This places build results into a board-specific directory rather than a
buildman-thread-specific directory. This is required so that we can
access the directory from test.py, and there's no risk of a particular
build's results being over-written by another build performed by the
same thread.

In theory, this can lead to slower builds when building many different
boards in a single buildman thread, since it removes the possibility of
incremental builds between boards. In practice however I didn't notice
longer build times when when enabling this option; if anything build
times decreased although I suspect that's simply due to general
variations in build performance across different machines within the
Travis CI infra-structure.

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