Ensure ${_CW} is uppercase when passing '-c' to mk-vmimage.sh,

otherwise with 'CLOUDWARE=vagrant-virtualbox', the path to the
configuration file may be incorrect.

MFC after:	3 days
Sponsored by:	The FreeBSD Foundation

release/Makefile.vm

@@ -59,7 +59,7 @@ cw-${_CW:tl}:
 		${.CURDIR}/scripts/mk-vmimage.sh \
 		-C ${.CURDIR}/tools/vmimage.subr -d ${.OBJDIR}/${.TARGET} \
 		-i ${.OBJDIR}/${_CW:tl}.img -s ${VMSIZE} -f ${${_CW:tu}_FORMAT} \
-		-S ${WORLDDIR} -o ${.OBJDIR}/${${_CW:tu}IMAGE} -c ${${_CW}CONF}
+		-S ${WORLDDIR} -o ${.OBJDIR}/${${_CW:tu}IMAGE} -c ${${_CW:tu}CONF}
 	touch ${.TARGET}
 
 cw${_CW:tl}-package: