Add: support for caching and unpacking the rootfs

This speeds up iterative development of a vmdb specification file: the
output of qemu-debootstrap and the apt installation additinal packages
are cached. They take up most of the time of a full run. If modifying
other parts of raspi3.yaml, this means an iteration drops from about
20 minutes to 16 seconds on my laptop.

If making changes to the cached parts, the cache tarball needs to be
deleted between runs.

The --rootfs-tarball options MUST now be used on all invocations.

raspi3.yaml

@@ -37,6 +37,8 @@ steps:
     dirname: '/boot/firmware'
     fs-tag: boot-fs
 
+  - unpack-rootfs: root-fs
+
   # We need to use Debian buster (currently testing) instead of Debian stretch
   # (currently stable) for:
   #
@@ -58,6 +60,7 @@ steps:
     - main
     - contrib
     - non-free
+    unless: rootfs_unpacked
 
   # TODO(https://bugs.debian.org/877855): remove this workaround once
   # debootstrap is fixed
@@ -65,6 +68,7 @@ steps:
     shell: |
       echo 'deb http://deb.debian.org/debian buster main contrib non-free' > /etc/apt/sources.list
       apt-get update
+    unless: rootfs_unpacked
 
   - apt: install
     packages:
@@ -78,6 +82,10 @@ steps:
     - raspi3-firmware
     - linux-image-arm64
     fs-tag: root-fs
+    unless: rootfs_unpacked
+
+  - cache-rootfs: root-fs
+    unless: rootfs_unpacked
 
   - shell: |
       echo "rpi3" > "${ROOT?}/etc/hostname"
@@ -122,6 +130,7 @@ steps:
   # the partition labeled raspiroot instead of forcing it to mmcblk0p2
   - chroot: root-fs
     shell: |
+      ls -aR /boot
       sed -i 's/.dev.mmcblk0p2/LABEL=RASPIROOT/' /boot/firmware/cmdline.txt
 
   # TODO(https://github.com/larswirzenius/vmdb2/issues/24): remove once vmdb