DMR: Fix Hotspot Power Savings setting by Al AF4FA

If you lose WIFI after a period of inactivity it is due
to the power-saving-mode of the WiFi-chipset is set to on.

To get rid of this problem, perform the following steps:
* First login to your Pi by ssh.
* verify current setting: by typing:

iw wlan0 get power_save

* This shows, that power-saving is currently on or off.
* If off you are done. If on do the following.
* make the root-partition writable:

sudo mount -o remount,rw /

* edit /etc/network/interfaces:

sudo nano /etc/network/interfaces

* scroll down to the section wlan0 (iface wlan0) after the line
* wpa-conf /etc/wpa_supplicant/wpa_supplicant.conf
* insert the following line:

wireless-power off

* Repeat this step for wlan1
* Save the file and reboot!

sudo reboot

* Connect by ssh again, and recheck power-saving:

iw wlan0 get power_save

* Now it should be off. If so you are done.