extra_script.py

Import("env")

# please keep $SOURCE variable, it will be replaced with a path to firmware

# Generic
env.Replace(
    UPLOADER="executable or path to executable",
    UPLOADCMD="$UPLOADER $UPLOADERFLAGS $SOURCE"
)

# In-line command with arguments
env.Replace(
    UPLOADCMD="executable -arg1 -arg2 $SOURCE"
)

# Python callback
def on_upload(source, target, env):
    print(source, target)
    firmware_path = str(source[0])
    # do something
    env.Execute("executable arg1 arg2")

env.Replace(UPLOADCMD=on_upload)