Merge the following from ^/projects/release-vmimage:
r272436, r272437, r272792:

r272436:
  Remove the first argument to panic(), which was initially
  intended to be the exit code, however when a non-zero exit
  code was returned to release/Makefile, this would prevent
  any remaining (and possibly successful) stages from being
  attempted.

r272437:

  If the vm-base target fails, prevent the vm-image target
  from being run since it cannot possibly succeed.

r272792:

  Add /usr/local/bin and /usr/local/sbin to PATH, needed
  if third-party software needs to use utilities outside
  of the base system during post-install stages (indexinfo
  is one culprit).

MFC after:	3 days
X-MFC-10.1:	yes
Sponsored by:	The FreeBSD Foundation