main/bootmac: support systemd

This adds a -systemd subpkg to install a unit file (for bluetooth right
now). This also includes some patches that were sent upstream to bootmac
project, which can be dropped if/when it's accepted in the bootmac
project and a new release is made.

The patches here have been sent upstream:
bootmac!10

openrc installs will still use the udev rule to run bootmac for
bluetooth, since that seems to work reliably (and there's no concept of
service targets or anything that would make this easy to implement on
openrc, but I digress).

Both systemd and openrc installs will continue to use udev rules for
wifi mac setting, I don't have a way to test that this works, sd unit
file can be created later if it improves the situation there.