Travis: Expose build dir as variable

Some travis QEMU tests can transfer files between the build directory
and the guest U-Boot instance. For that to work, both need to have access
to the same directory.

This patch puts the current build path into an environment variable, so
that the environment generating python scripts can extract it from there
and read the respective files.

Signed-off-by: Alexander Graf <agraf@suse.de>
Reviewed-by: Tom Rini <trini@konsulko.com>