travis: Split the building into two parts

Buildman is used in two ways:

- to build a selection of boards (with no testing)
- to build a single board (and run pytest)

The gitlab and azure scrips do this in separate places, but travis does
not. To aid the refactoring process and keep the following patches in sync
across all three environments, split the code out in travis as well.

Use the buildman -w option for the single board. It is easier to
understand since it specifies the output directory directly. Also it
avoids needing to look at the internal .bm-work directory.

This initially creates some duplicate code, but by the end of the series
we have two completely different build paths with different arguments.

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