add bp-tools support for plain adb mode

On userdebug builds, only adb is brought up, not adb+mtp.
When booting into bp-tools mode on a userdebug device, no overlay
entry is present, which means the expected mode isn't entered.

Add an overlay entry.

Bug: 22062484
Change-Id: I2dd3b6064c2591dca595a216df9a37a11eab793c
1 file changed