travis: add packages for UEFI secure boot test

Pytest for UEFI secure boot will use several host commands.
In particular, Test setup relies on efitools, whose version must be v1.5.2
or later. So fetch a new version of deb package directly.
Please note it has a dependency on mtools, which must also be installed
along wih efitools.

In addition, the path, '/sbin', is added to PATH for use of sgdisk and
mkfs.

Signed-off-by: AKASHI Takahiro <takahiro.akashi@linaro.org>
Reviewed-by: Tom Rini <trini@konsulko.com>