mfi-common: Fall back to anointed builds in Executive mode

mfi-common: Fall back to anointed builds in Executive mode

Previously, `make-*-flight' would not work unless FREEBSD_*_BUILDJOB
was set.  Now we use the anointed values if we can find them.

If we can't, mg-anoint retrieve will print a warning.

Signed-off-by: Ian Jackson <Ian.Jackson@eu.citrix.com>
CC: Roger Pau Monné <roger.pau@citrix.com>