Allow udev-hid-bpf binary to regenerate the hwdb

Currently the hwdb is generated as part of build.rs. This works fine as long as we use this project to regenerate everything.

However, the ideal process would be for a user to drop a file in /usr/local/lib/firmware/hid, regenerate the hwdb (udev-hid-bpf hwdb update) and then have the new file included in the hwdb.

This will allow to build the bpf sources from outside of this repository (in the kernel directory) and also generate the hwdb without too much pain.

This should also allow distributions to ship udev-hid-bpf and regenerate the hwdb in the post-install to take into account the users' files.

IMO we should extract the build.rs part that does that in a separate file, and either include that file directly in build.rs or directly call udev-hid-bpf hwdb update in the install process.