conf: Add usbhid and hidbus to GENERIC* kernel configs

Include the new unified HID stack by default in generic.
This will allow us to migrate to the multi-stack hkbd and hms instead of
relying on the older ukbd and ums which only work with USB.
To test those drivers just add hw.usb.usbhid.enable=1 in loader.conf

Differential Revision: https://reviews.freebsd.org/D45658
Reviewed by: emaste, imp, wulf (all older version)
Sponsored by: Beckhoff Automation GmbH & Co. KG