flash: Add option to specify image URL

The `flash.zsh` script now takes an optional `--image-url` argument,
which can be used to specify a different FCOS base image.  This could be
to use a custom image or to simply avoid downloading the same image from
the Internet repeatedly.

flash.zsh

@@ -67,10 +67,15 @@ function hybridize_gpt() {
 function install_coreos() {
     local ignition="$1"
     local dev="$2"
+    local url="$3"
 
-    coreos-installer install \
-        -a aarch64 \
-        -s stable \
+    if [ -z "${url}" ]; then
+        set -- -a aarch64 -s stable
+    else
+        set -- --image-url "${url}"
+    fi
+
+    coreos-installer install "$@" \
         -i "${ignition}" \
         --console ttyS0,115200n8 \
         "${dev}"
@@ -79,6 +84,7 @@ function install_coreos() {
 
 function parse_args() {
     pi=4
+    image_url=
     while [ $# -gt 0 ]; do
         case "$1" in
         --pi)
@@ -88,6 +94,13 @@ function parse_args() {
         --pi=*)
             pi=${1#--pi=}
             ;;
+        --image-url)
+            shift
+            image_url=${1}
+            ;;
+        --image-url=*)
+            image_url=${1#--image-url=}
+            ;;
         *)
             if [ -z "${ignition-}" ]; then
                 ignition="${1}"
@@ -128,7 +141,7 @@ if [ $(id -u) -ne 0 ]; then
     exec sudo "$0" "$@"
 fi
 
-install_coreos "${ignition}" "${dev}"
+install_coreos "${ignition}" "${dev}" "${image_url}"
 
 case "${pi}" in
 2|3)