deploy: Support an empty /etc and populated /usr/etc

[cgwalters]

In preparation for support for a transient /etc: #2868
particularly in combination with composefs.

Basically it's just much more elegant if we can directly mount an overlayfs on the empty etc directory, using usr/etc as the lower.

In the composefs case, we'd have to mount the composefs overlayfs itself writable (and call mkdir) just so we can make that empty etc directory which is ugly.

[ericcurtin]

LGTM

[alexlarsson]

For the record, we always add an /etc to the composefs image, so this is not needed in the ostree commit for composefs + transient-etc to work.

See ostree_repo_checkout_composefs() which has:

  char *root_dirs[] = { "usr", "etc", "boot", "var", "sysroot" };

  /* To work as a rootfs we need some root directories to use as bind-mounts */
  for (i = 0; i < G_N_ELEMENTS (root_dirs); i++)

[cgwalters]

@alexlarsson
I didn't understand that code at all and I meant to circle back to it.

I find it confusing to have these hardcoded directories. Why don't we just require them to be in the ostree commit as we were doing before?