Fix local testnet scripts (#2229)

## Issue Addressed

Resolves #2094 

## Proposed Changes

Fixes scripts for creating local testnets. Adds an option in `lighthouse boot_node` to run with a previously generated enr.

scripts/local_testnet/bootnode.sh

@@ -0,0 +1,33 @@
+#!/usr/bin/env bash
+
+#
+# Generates a bootnode enr and saves it in $TESTNET/boot_enr.yaml
+# Starts a bootnode from the generated enr.
+#
+
+source ./vars.env
+
+echo "Generating bootnode enr"
+
+lcli \
+	generate-bootnode-enr \
+	--ip 127.0.0.1 \
+	--udp-port $BOOTNODE_PORT \
+	--tcp-port $BOOTNODE_PORT \
+	--genesis-fork-version $GENESIS_FORK_VERSION \
+	--output-dir $DATADIR/bootnode
+
+bootnode_enr=`cat $DATADIR/bootnode/enr.dat`
+echo "- $bootnode_enr" > $TESTNET_DIR/boot_enr.yaml
+
+echo "Generated bootnode enr and written to $TESTNET_DIR/boot_enr.yaml"
+
+DEBUG_LEVEL=${1:-info}
+
+echo "Starting bootnode"
+
+exec lighthouse boot_node \
+    --testnet-dir $TESTNET_DIR \
+    --port $BOOTNODE_PORT \
+    --listen-address 127.0.0.1 \
+    --network-dir $DATADIR/bootnode \