Add initramfs support to write extensions that produce disks

If INITRAMFS_PATH is specified and the file exists, then the produced
kernel command line will use root=UUID=$uuid_of_created_disk rather than
root=/dev/sda, which may be incorrect.

Help files have been updated to mention the new option.

This leads to an unfortunate duplication of the path to the initramfs,
in both the location field of the nested deployment and the
INITRAMFS_PATH of the disk image creation.

However, an initramfs could be produced by a chunk and put in the same
place, so it doesn't make sense to couple the rawdisk and initramfs
write extensions to remove this duplication.

Similarly, there may be multiple valid initramfs in the rootfs e.g.
extlinux loads a hypervisor, which is Linux + initramfs, and the
initramfs then boots a guest Linux system, which uses a different
initramfs.

This makes it important to explicitly let the rootfs write extensions
know which to use, or not as the case may be.

util-linux's blkid is required, since the busybox version ignores the
options to filter its output, and parsing the output is undesirable.

Because syslinux's btrfs subvolume support is limited to being able to
use a non-0 default subvolume, the initramfs has to be copied out of
the run-time rootfs subvolume and into the boot subvolume.

This pushed the required disk space of a minimal system over the 512M
threshold because we do not have the userland tooling support to be able
to do a btrfs file contents clone.

1 files changed, 4 insertions, 0 deletions

diff --git a/virtualbox-ssh.write.help b/virtualbox-ssh.write.help
new file mode 100644
index 00000000..8b5053a5
--- /dev/null
+++ b/virtualbox-ssh.write.help
@@ -0,0 +1,4 @@
+help: |
+    The INITRAMFS_PATH option can be used to specify the location of an
+    initramfs for syslinux to tell Linux to use, rather than booting
+    the rootfs directly.