Set a default hostname for virtual machine images.

A recent bug in security/sudo causes segmentation faults when
the system is not configured with a hostname, which causes issues
with some virtual machine setups, notably Vagrant.  Set the default
hostname to the output of 'uname -o'.

Submitted by:	Nicholas Fiorentini
Sponsored by:	The FreeBSD Foundation

release/tools/vmimage.subr

@@ -140,6 +140,10 @@ vm_install_base() {
 			>> ${DESTDIR}/etc/fstab
 	fi
 
+	local hostname
+	hostname="$(echo $(uname -o) | tr '[:upper:]' '[:lower:]')"
+	echo "hostname=\"${hostname}\"" >> ${DESTDIR}/etc/rc.conf
+
 	mkdir -p ${DESTDIR}/dev
 	mount -t devfs devfs ${DESTDIR}/dev
 	chroot ${DESTDIR} /usr/bin/newaliases