travis-ci: build Tegra boards

ARMv7 Tegra boards aren't currently covered by any other travis-ci jobs.
Add a new job to build them.

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