samd21 support

SAMD21 doesn't appear to be supported. I get this:

`matt 2013:(master)$ ./flash.js --port /dev/ttyACM1 --reset -D
> N#
> w400e0940,#
chipId: 0x0
> V#
vStr: v2.0 [Arduino:XYZ] May  6 2019 14:48:36

FAILED: TypeError: Cannot read property 'init' of null
`

Do I need to add a new clause to EEFCFactory?