make-flight: Support specifying a mini-os tree+revision

make-flight: Support specifying a mini-os tree+revision

This is useful for standalone or adhoc use as well as (presumably)
bisection.

There is no ap-* or cr-daily-* integration here because I didn't need
it (i.e. I'm not intending to create a new mini-os branch here).

In order to cope with Xen <= 4.5 where extras/mini-os exists but is
part of xen.git and not something cloned from elsewhere add a
$optional argument (itself optional) to dir_identify_vcs which if true
causes dir_identify_vcs to return 'none' instead of failing.

Previously dir_identify_vcs failed with:
    bash: line 5: fail: command not found
because the fail command is undefined. Instead echo fail and use that
to trigger the $optional handling.

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>