Allow for default values

[not-a-doctor-stromberg]

Copied from PNDeb/pinenote-debian-image#17

Hi Maximilian,

My preferred driver settings are:

rockchip_ebc.bw_mode=1 rockchip_ebc.default_waveform=1 rockchip_ebc.refresh_threshold=30 rockchip_ebc.auto_refresh=1
I tried setting those in extlinux.conf, but the bw_mode at least is overwritten by the gnome helper here.

It seems like in other places we read the file first with _get_content (in _add_auto_refresh_button for example) -- I will try doing something similar for changing bw mode (and will try to clean up the spacing as well).

Let me know if you disagree with this approach. I am planning on issuing a pull request for this, but if someone beats me to it, no problamo :)