usb: rename board_usb_init_type to usb_init_type

This will be used by usb_lowlevel_init so it will
no longer be used by only board specific functions.

Move definition of enum usb_init_type higher in file
so that it will be available for usb_low_level_init.

Signed-off-by: Troy Kisky <troy.kisky@boundarydevices.com>