Reduce avrdude verbosity

Previously, when verbose uploads were enabled, avrdude was run with four
-v options, causing it to dump all raw bytes exchanged with the
bootloader. This floods the console so much that meaningful output
mostly disappears.

Most users probably want to enable verbose mode just to see what avrdude
command is ran. Furthermore, users that benefit from the raw bytes
dumped are perfectly capable of either running avrdude manually, or
modifying platform.txt. Given that, running avrdude with just one  -v
should be plenty.

This fixes #891.