Add base64 tarball target

The tarball can be copied to the VM through the serial port with the
following command:
    # cat > guix-installer-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 guix-installer-vm.tar.xz.b64 > guix-installer-vm.tar.xz
    # tar xf guix-installer-vm.tar.xz

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

.gitignore

@@ -4,6 +4,7 @@ guix-commit.txt
 guix-installer-vm.img
 guix-installer-vm.tar
 guix-installer-vm.tar.xz
+guix-installer-vm.tar.xz.b64
 id_ed25519
 id_ed25519.pub
 signing-key.pub

Makefile

@@ -41,6 +41,9 @@ guix-installer-vm.tar: $(TARBALL_SOURCE)
 guix-installer-vm.tar.xz: guix-installer-vm.tar
 	xz -f -9e --verbose $<
 
+guix-installer-vm.tar.xz.b64: guix-installer-vm.tar.xz
+	base64 $< > $@
+
 IMAGE_SOURCE = \
 	$(TARBALL_SOURCE) \
 	guix-installer-vm.tar.xz