Makefile.vm: Fix duplicate rc.conf files
Two bugs in Makefile.vm resulted in disk images being "built" multiple
times, resulting in lines added to /etc/rc.conf being duplicated:
- The vm-image target reused the same "staging tree" directory for all
of its builds (multiple disk image types and multiple filesystem types).
- The cw-type-flavour-fs target depends on emulator-portinstall, which
did not have a 'touch ${.TARGET}' and thus re-ran every time -- and
caused the cw-type-flavour-fs target to be re-run. This was triggered
by release builds running make cloudware-release (creating the disk
images) followed by make ec2amis (which re-created the disk images
prior to uploading them).
MFC After: 1 week
Sponsored by: https://www.patreon.com/cperciva
(cherry picked from commit 97bd53ef4d20b7d15e0b0976e885af9438f5637e)