Added usb_vendors.* which handles creating (and deleting) a list of vendor ids.
This list is meant to be used everywhere the built-in lists (usb_osx), or the
built-in vendor IDs (transport_usb) were used.
For now the list is only built with the built-in VENDOR_ID_*. Next step
is to read a small file created from all the SDK add-on.
Other misc changes: made is_adb_interface present only if ADB_HOST is true
to prevent accessing a list that doesn't exist (usb_vendors is only
compiled for the host version of adb).
Xavier Ducrohet