exit: fix no /usr/bin/env error

if you execute the bootstrap stage with no internet connection, you get
the following output:

```
[2020-03-10 19:18:46] lb bootstrap
P: Setting up clean exit handler
[2020-03-10 19:18:46] lb bootstrap_cache restore
[2020-03-10 19:18:46] lb bootstrap_debootstrap
P: Begin bootstrapping system...
P: If the following stage fails, the most likely cause of the problem is with your mirror configuration or a caching proxy.
P: Running debootstrap (download-only)...
I: Retrieving InRelease
I: Retrieving Release
E: Failed getting release file http://deb.debian.org/debian/dists/buster/Release
P: Begin unmounting filesystems...
P: Saving caches...
chroot: failed to run command ‘/usr/bin/env’: No such file or directory
```

the last line looked suspicious. investigating it turns out that there was
a deficiency in the exit handler.

when debootstrap fails to download what it needs due to lack of a
connection, that failure due to `set -e` causes the Exit() handler to kick
in. Part of this includes outputting the "Saving caches..." line, before
then making a call to Save_package_cache(). That in turn runs the following
command:
```
Chroot chroot "apt-get autoclean" || true
```
The Chroot() function includes a line starting with:
```
${_LINUX32} chroot "${CHROOT}" /usr/bin/env
```
which is the source of the last output line.

the reason we see this unexpected output is that with bootstrapping having
failed, there is no /usr/bin/env within the chroot so it is bound to fail.

the fact is, the exit handler has no business trying to pretty much
anything that it does if the bootstrap_debootstrap stage has not
completed.

this implements such a restriction and thus resolves the problem of this
unexpected and confusing output in the described situation.

we will now see:
```
[2020-03-10 19:18:46] lb bootstrap
P: Setting up clean exit handler
[2020-03-10 19:18:46] lb bootstrap_cache restore
[2020-03-10 19:18:46] lb bootstrap_debootstrap
P: Begin bootstrapping system...
P: If the following stage fails, the most likely cause of the problem is with your mirror configuration or a caching proxy.
P: Running debootstrap (download-only)...
I: Retrieving InRelease
I: Retrieving Release
E: Failed getting release file http://deb.debian.org/debian/dists/buster/Release
```

functions/exit.sh

@@ -18,6 +18,12 @@ Exit ()
 		set | grep -e ^LB
 	fi
 
+	# Skip if we have not yet completed the initial bootstrapping (bootstrap_debootstrap)
+	# (nothing to be done; avoids unhelpful messages)
+	if [ ! -e .build/bootstrap ]; then
+		return ${VALUE}
+	fi
+
 	# Always exit true in case we are not able to unmount
 	# (e.g. due to running processes in chroot from user customizations)
 	Echo_message "Begin unmounting filesystems..."