Implement USB keyboard support (based on patch by Marco Gerards)
* conf/i386-pc.rmk (pkglib_MODULES): Add `usb_keyboard.mod'.
(usb_keyboard_mod_SOURCES, usb_keyboard_mod_CFLAGS)
(usb_keyboard_mod_LDFLAGS): New variables.
* term/usb_keyboard.c: New file.
