Support flash from local files for auto_flash_from_pvt.sh.

added `-l | --local` option
added new variable `DL_DIR` which rule is `pvt/{DEVICE_NAME}/{VERSION_NAME}/{USR_ENG}/{BUILD_ID}`

The latest build will always download from web, due to we don't know when the latest build be changed.

for #46 
Part 1: pvt done!
twci also need to create latest folder to store the downloaded build.