[ev3dev-jessie] Exclude dummy ssh host keys from tar image.

Also don't have to delete them now before creating new ones on first boot.
[ev3dev-jessie] Disable NFS services by default.
Use -r option for rootfs in proot

Fixes ev3dev/brickstrap#1
ev3dev/ev3dev#209
Set debian_chroot when running shell.

This will cause the prefix "(brickstrap)" to be shown in front of the bash
prompt if the user has not removed it from their ~/.bashrc.
[ev3dev-jessie] Don't enable USB Gadget by default.

We can easily enable it in brickman now.
[ev3dev-jessie] Add libnss-myhostname wanted by systemd.
update changelog for v0.2.7