Clarify/ Update Instructions for Ubuntu

[thetoivonen]

The test sketch that came on the device works as expected, and the device passes the tests. I cannot upload any other sketches.

According to the README:

Flash the binary directly or via Arduno IDE sudo bossac --port=ttyUSB0 -e -w -v -b /tmp/CANFDuino_Test500kb.ino.CANFDuino.bin

After placing the downloaded bossa binary in the folder as instructed and restarting the arduino IDE:

Upload fails with:
No device found on ttyUSB0 Failed uploading: uploading error: exit status 1

Using the bossac1.7.0 binary for manual upload fails with:
sudo ./bossac --port=ttyUSB0 -e -w -v -b ~/Desktop/CANFDuino_SavvyCAN.ino.bin
No device found on ttyUSB0

Using bossac1.9.1 compiled locally fails with:
sudo ./bossac --port=ttyUSB0 -e -w -v -b ~/Desktop/CANFDuino_SavvyCAN.ino.bin
Device unsupported

Trying to set the port speed before uploading does not help:
sudo stty -F /dev/ttyUSB0 1200; sudo ./bossac --port=ttyUSB0 -e -w -v -b ~/Desktop/CANFDuino_SavvyCAN.ino.bin
No device found on ttyUSB0

It seems that the device is going back to a baud rate of 115200 before the upload can complete.