Switch from gradle to maven

Maven is already used in the main OSv build, so it's kind of straight-forward
that it would be used here as well.

This patch:

* Converts the build tool from gradle to maven
* Adjusts the OSv module configurations to match
* Updates .gitignore
* Remove everything related to gradle

Reviewed-by: Tomasz Grabiec <tgrabiec@gmail.com>
Signed-off-by: Or Cohen <orc@fewbytes.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>