MFC r288341, r288345, r288347:
r288341:
Honor VMFORMATS and VMSIZE if set in release.conf. [1]
r288345:
In followup to r288341, ensure VMFORMATS and VMSIZE are not set to empty values, which would result in nonintuitive build errors.
r288347:
Append VMFORMATS and VMSIZE to RELEASE_RMAKEFLAGS only if WITH_VMIMAGES is set.
PR: 203420 [1]
Sponsored by: The FreeBSD Foundation