Don't build ota files for generic device

http://b/issue?id=2535706
Extend from product name generic to device name generic.
Generic device does not have files such as kernel which are necessary
to build ota files.
Remove test of aosp_emulator_us, which is in no use any more.

Change-Id: I46025125c152fba79e33092dea817a3be7dc0f91