Preparation work for adb to support USB vendor Ids provided by SDK add-ons.

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).
diff --git a/adb/adb.h b/adb/adb.h
index aebb81a..7762e00 100644
--- a/adb/adb.h
+++ b/adb/adb.h
@@ -375,7 +375,9 @@
 void usb_kick(usb_handle *h);
 
 /* used for USB device detection */
+#if ADB_HOST
 int is_adb_interface(int vid, int pid, int usb_class, int usb_subclass, int usb_protocol);
+#endif
 
 unsigned host_to_le32(unsigned n);
 int adb_commandline(int argc, char **argv);