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:

1. The vm-image target reused the same "staging tree" directory for all
of its builds (multiple disk image types and multiple filesystem types).

2. 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

release/Makefile.vm

@@ -81,6 +81,7 @@ emulator-portinstall:
 	env ASSUME_ALWAYS_YES=yes pkg install -y emulators/qemu-user-static
 .endif
 .endif
+	touch ${.TARGET}
 
 QEMUSTATIC=/usr/local/bin/qemu-${TARGET_ARCH}-static
 .endif
@@ -137,10 +138,11 @@ vm-image:
 .if defined(WITH_VMIMAGES) && !empty(WITH_VMIMAGES)
 . for FORMAT in ${VMFORMATS}
 .  for FS in ${VMFSLIST}
-	mkdir -p ${.OBJDIR}/${.TARGET}
+	mkdir -p ${.OBJDIR}/${.TARGET}-${FORMAT}-${FS}
 	env TARGET=${TARGET} TARGET_ARCH=${TARGET_ARCH} SWAPSIZE=${SWAPSIZE} \
 		${.CURDIR}/scripts/mk-vmimage.sh \
-		-C ${.CURDIR}/tools/vmimage.subr -d ${.OBJDIR}/${.TARGET} -F ${FS} \
+		-C ${.CURDIR}/tools/vmimage.subr \
+		-d ${.OBJDIR}/${.TARGET}-${FORMAT}-${FS} -F ${FS} \
 		-i ${.OBJDIR}/${FORMAT}.${FS}.img -s ${VMSIZE} -f ${FORMAT} \
 		-S ${WORLDDIR} -o ${.OBJDIR}/${VMBASE}.${FS}.${FORMAT}
 .  endfor