add flash script for linux, to copy with assembly artefacts via CI

scripts/flash_nix.sh

@@ -0,0 +1,29 @@
+#!/bin/sh
+
+cd $(dirname "$0")
+
+FLASH_COMMAND="$(cat flash.command)"
+
+if [ -n "$1" ]; then
+    if [ -c "$1" ]; then
+        PORT="$1"
+
+        TOOL="$(echo "$FLASH_COMMAND" | sed -e 's/\s.*$//')"
+        ESPTOOL=`which $TOOL`
+        if [ -n "$ESPTOOL" ] && [ -x "$ESPTOOL" ]; then
+            echo "$FLASH_COMMAND" | sed -e "s|(PORT)|$PORT|g" | sh -x
+            exit $?
+        else
+            echo "Can't find executable $ESPTOOL"
+            echo
+        fi
+    else
+        echo "Invalid port $1"
+        echo
+    fi
+fi
+
+echo "Usage: $0 <serial_port>"
+echo "Available ports:"
+find /dev/tty* | grep -E "ACM|USB" --color=never
+exit 255;