flash: Add option to override console spec

The `flash.zsh` script now takes an optional `--console` argument, which
can be used to override the `console=` kernel command line argument.

flash.zsh

@@ -68,16 +68,19 @@ function install_coreos() {
     local ignition="$1"
     local dev="$2"
     local url="$3"
+    local console="$4"
 
     if [ -z "${url}" ]; then
         set -- -a aarch64 -s stable
     else
         set -- --image-url "${url}"
     fi
+    if [ -n "${console}" ]; then
+        set -- "$@" --console "${console}"
+    fi
 
     coreos-installer install "$@" \
         -i "${ignition}" \
-        --console ttyS0,115200n8 \
         "${dev}"
     sync; sync; sync
 }
@@ -85,6 +88,7 @@ function install_coreos() {
 function parse_args() {
     pi=4
     image_url=
+    console=ttyS0,115200n8
     while [ $# -gt 0 ]; do
         case "$1" in
         --pi)
@@ -101,6 +105,13 @@ function parse_args() {
         --image-url=*)
             image_url=${1#--image-url=}
             ;;
+        --console)
+            shift
+            console=${1}
+            ;;
+        --console=*)
+            console=${1#--console=}
+            ;;
         *)
             if [ -z "${ignition-}" ]; then
                 ignition="${1}"
@@ -141,7 +152,7 @@ if [ $(id -u) -ne 0 ]; then
     exec sudo "$0" "$@"
 fi
 
-install_coreos "${ignition}" "${dev}" "${image_url}"
+install_coreos "${ignition}" "${dev}" "${image_url}" "${console}"
 
 case "${pi}" in
 2|3)