generate keys in dropbear sysvinit script

buildroot upstream now uses '-R' to get their keys generated
which is to generate them on the fly.  We'd rather they're explicitly
generated ahead of time so that we can log them.
Index: buildroot/package/dropbear/S50dropbear
===================================================================
--- buildroot.orig/package/dropbear/S50dropbear	2014-09-01 11:20:56.000000000 +0000
+++ buildroot/package/dropbear/S50dropbear	2014-09-16 19:29:54.320096000 +0000
@@ -6,9 +6,17 @@
 # Allow a few customizations from a config file
 test -r /etc/default/dropbear && . /etc/default/dropbear
 
+DROPBEAR_KEYTYPES="rsa dss ecc"
 start() {
 	DROPBEAR_ARGS="$DROPBEAR_ARGS -R"
 
+	local ktype file
+	for ktype in rsa dss ecdsa; do
+		file="/etc/dropbear/dropbear_${ktype}_host_key"
+		cirros-per instance dropbear-keygen-$ktype -- \
+			dropbearkey -t "$ktype" -f "$file" >/dev/null 2>&1 ||
+		echo "WARN: generating key of type $ktype failed!"
+	done
 	echo -n "Starting dropbear sshd: "
 	umask 077
 	start-stop-daemon -S -q -p /var/run/dropbear.pid \