planet-geminipda: add support for Planet Gemini PDA based on earlier work

This MR attempts to integrate Gemini PDA support into master from old device-planet-geminipda branch (https://github.com/postmarketOS/pmbootstrap/commits/device-planet-geminipda). Most of hybris-related changes (besides KWin) was merged, so it is just device and kernel aports now.

The state of hardware support is unchanged from https://wiki.postmarketos.org/wiki/Planet_Computers_Gemini_PDA_(planet-geminipda). Without hybris, it can boot to graphics with X11 fbdev driver, the included config enables correct rotation (as it has portrait display, but usable only in landscape).

For running it with hybris, which can allow for 3D acceleration, we need a way to package and provide large (http://sfos.scanf.su/geminipda/system_20181124.tar.xz) LXC image for running Halium Android container. Lately Adam Boardman from Gemini PDA community also got voice calls working in Debian install with a mix of SailfishOS middleware and Ubuntu Touch applications, which could be probably adapted for postmarketOS if needed.

device/device-planet-geminipda/10-geminipda-loopback-device.sh

+#!/bin/sh
+. /etc/deviceinfo
+. ./init_functions.sh
+
+mount_loopback_device() {
+	loopback_img=postmarketOS.img
+	partitions="/dev/mmcblk0p29 /dev/mmcblk0p30 /dev/mmcblk0p31"
+
+	mkdir /tmpmnt
+	for part in $partitions; do
+		mount -o ro $part /tmpmnt
+
+		if [ -f "/tmpmnt/$loopback_img" ]; then
+			mount -o remount,rw /tmpmnt
+			loopback_device=$(losetup -f)
+			losetup $loopback_device "/tmpmnt/$loopback_img"
+			kpartx -afs $loopback_device
+			return 0
+		fi
+
+		umount /tmpmnt
+	done
+}
+
+mount_loopback_device