Make swap size configurable (#3882)

Allow configuration of the swap size via /etc/default/haos-swapfile file. By
setting the SWAPSIZE variable in this file, swapfile get recreated on the next
reboot to the defined size. Size can be either in bytes or with optional units
(B/K/M/G, accepting some variations but always interpreted as power of 10). The
size is then rounded to 4k block size. If no override is defined or the value
can't be parsed, it falls back to previously used 33% of system RAM.

Fixes #968

buildroot-external/rootfs-overlay/usr/lib/systemd/system/etc-default.mount

@@ -0,0 +1,13 @@
+[Unit]
+Description=Persistent /etc/default directory
+Requires=mnt-overlay.mount
+After=mnt-overlay.mount
+
+[Mount]
+What=/mnt/overlay/etc/default
+Where=/etc/default
+Type=None
+Options=bind
+
+[Install]
+WantedBy=hassos-bind.target