[SAMD21] Don't issue a chip erase if address is 0

If a user selects "Nano33BLE" in the IDE but tries to flash a SAMD board, the commandline being used will not specify a base address, so the whole flash will be erased (the bootloader too)

Use git describe for the bossa version

Added SAM Cortex families to the device reset.

Fixed some minor OSX build issues.

Removed parallel build from mingw to avoid lockups

Fixed progress bar

Upped version to 1.6-arduino

Added fast verify command support

The new command uses CRC to perform very fast block compare without the need
to transfer the block back and forth.

Ignore compile error: unused-but-set-variable