Issue
when try vta on fpga board, would see a Infinite recursive
device_api.ext_dev issue that cause stack overflow and vta
failed.

Analysis:
device_api.ext_dev function in rpc_server.py is use to load
vta library, once vta library get load, device_api.ext_dev would
get replaced with vta function by vta library, vta device_api.cc
did such work, but because a logic issue in VTA.cmake, the said file
not get compiled, then vta would keep failing on rpc_server.py.

Solution:
fix the logic issue in VTA.cmake.