scripts: west flash with stm32CubeProgrammer and ext-loader

Add the option to support an external loader for flashing
hex file to internal and external NOR flash using
the STM32CubProgrammer CLI with a  board_runner_args
"--extload=MX25LM51245G_STM32U585I-IOT02A.stldr"
The absolute path of the stldr file is added to the
stm32CubeProgrammer command.

Signed-off-by: Francois Ramu <francois.ramu@st.com>