vmimage.subr: Redirect etcupdate log to stdout

While we're here, send the etcupdate log from generating base.txz
to stdout instead of /dev/null (see e972e408d19a) as well.

Reviewed by: emaste, gordon, jrtc27
MFC after: 1 week
Sponsored by: Amazon
Differential Revision: https://reviews.freebsd.org/D48719

(cherry picked from commit ace4637ee0c666c6f5a1c664f8956f7302234edd)