Allow devices to override USB init

This enables devices to provide their own USB configuration. The
contents of init.recovery.usb.rc can most simply be modified and
included in a device's init.recovery.${ro.hardware}.rc.

Use option:
TW_EXCLUDE_DEFAULT_USB_INIT := true

Rationale: Some devices handle USB pid switching in the kernel. In this
case, USB init provides switching functions which differ from the
standard on property:sys.usb.config=xyz. Other devices should not see
USB attempt mtp,adb mode when TW_EXCLUDE_MTP is set.

Cherry-picked from android-5.0

Change-Id: Ief0fcaf46a1782102166fc1b733a34b1a1ba0802