cmdlib: mount src/config-git into supermin instead of src/config if present

[jlebon]

The real repo is in src/config-git in a subdir setup and src/config
is just a relative symlink to the subdir in question.

This should allow us to use subdirs in the CI of openshift/os for
different RHEL/CentOS versions.

Long-term, we should probably move away from subdirs into something more
structured that doesn't require symlinking everything into the subdirs.

[jlebon]

I tested this with FCOS locally using:

diff --git a/mysubdir/fedora.repo b/mysubdir/fedora.repo
new file mode 120000
index 00000000..6e70b00b
--- /dev/null
+++ b/mysubdir/fedora.repo
@@ -0,0 +1 @@
+../fedora.repo
\ No newline at end of file
diff --git a/mysubdir/image-base.yaml b/mysubdir/image-base.yaml
new file mode 120000
index 00000000..c6c81978
--- /dev/null
+++ b/mysubdir/image-base.yaml
@@ -0,0 +1 @@
+../image-base.yaml
\ No newline at end of file
diff --git a/mysubdir/image.yaml b/mysubdir/image.yaml
new file mode 120000
index 00000000..73c5b031
--- /dev/null
+++ b/mysubdir/image.yaml
@@ -0,0 +1 @@
+../image.yaml
\ No newline at end of file
diff --git a/mysubdir/live b/mysubdir/live
deleted file mode 120000
index e69de29b..00000000
diff --git a/mysubdir/manifest.yaml b/mysubdir/manifest.yaml
new file mode 100644
index 00000000..a89f57e2
--- /dev/null
+++ b/mysubdir/manifest.yaml
@@ -0,0 +1 @@
+include: ../manifest.yaml
diff --git a/mysubdir/overlay.d b/mysubdir/overlay.d
new file mode 120000
index 00000000..bdc0a4ca
--- /dev/null
+++ b/mysubdir/overlay.d
@@ -0,0 +1 @@
+../overlay.d
\ No newline at end of file
diff --git a/mysubdir/tests b/mysubdir/tests
deleted file mode 120000
index e69de29b..00000000

And then:

$ export FORCE_UNPRIVILEGED=1
$ cosa init ~/Code/gh/c/fcos mysubdir
$ cosa fetch
Using manifest: /srv/fcos/worktrees/test/src/config/manifest.yaml
...

[jlebon]

Closing in favour of #2934.