Add base64 tarball target

The tarball can be copied to the VM through the serial port with the
following command:
    # cat > mumble-vm.tar.xz.b64
the user then pastes the base64 content and types ctrl+d and this
results in the file being written.

The content can then be extracted with the following commands:
    # base64 -d mumble-vm.tar.xz.b64 > mumble-vm.tar.xz
    # tar xf mumble-vm.tar.xz

Signed-off-by: Denis 'GNUtoo' Carikli <GNUtoo@cyberdimension.org>

.gitignore

@@ -17,6 +17,7 @@ Makefile.in
 missing
 mumble-vm.tar
 mumble-vm.tar.xz
+mumble-vm.tar.xz.b64
 mumble-vm-machine.scm
 mumble-vm-system.scm
 signing-key.pub

Makefile.am

@@ -96,5 +96,8 @@ mumble-vm.tar: $(TARBALL_SOURCE)
 mumble-vm.tar.xz: mumble-vm.tar
 	xz -f -9e --verbose $<
 
+mumble-vm.tar.xz.b64: mumble-vm.tar.xz
+	base64 $< > $@
+
 deploy: $(IMAGE_SOURCE)
 	guix deploy -L . mumble-vm-machine.scm