Hardware bringup updates

Updates for bringing up Piksi hardware:
* Add function in AcqResult class to return the mean of the best acq SNR for all acquired satellites.
* Add option in Flash.erase_sector arguments to not check for restricted sectors.
* Refactor code that downloads index and firmware files from download.swiftnav.com.
This is in case the console for some reason the console can't download the index file, and thus can't tell if the user's firmware is out of date / download the latest firmware files - manually checking the website is the last resort.