USB gadget: Drop g_multi in favor of librem5-base

Draft, because I can't get the .deb to build and test it properly.

Draft, because the MAC should be done as a systemd enviroment variable, to prevent resetting at every upgrade.

Draft, because it's based on !220 (merged)

debian/librem5-devkit-base.postinst

@@ -21,8 +21,11 @@ case "$1" in
     configure)
 	# Enable serial console over USB
 	systemctl enable serial-getty@ttyGS0
-	# Mass storage is required for Ethernet+serial over USB via g_multi
-	truncate -s 1M /var/lib/mass_storage_dummy
+
+	# Mass storage is no longer required for Ethernet+serial over USB via g_multi
+    if md5sum /var/lib/mass_storage_dummy | grep -qs '^b6d81b360a5672d80c27430f39153e2c '; then
+	    rm /var/lib/mass_storage_dummy
+	fi
 
 	if [ -z "$2" ]; then
 	  echo "Initial install, linking shipment script"