Add separate configuration partition for core-image-brewerycontrol

[jonte]

This allows re-flashing the root partition without overwriting configuration.

Configuration which needs to be moved to this partition includes:

• tempserver (currently /etc/tempserver.conf)
• brewerycontrol_qt (currently /etc/brewerycontrol.conf)
• wifi settings (currently /etc//wpa_supplicant/wpa_supplicant-wlan0.conf)

For tempserver and brewerycontrol_qt, a build-time directive could be added to set a "non-standard" configuration path. For wifi, the wpa_supplicant service file likely needs to be changed.