boards: stm32l5: Update TFM related flashing instructions

Following the work done to add TFM support on b_u585i_iot02a,
update other STM32 TFM enabled boards to benefit from the
progress made on TFM flashing instructions.

Signed-off-by: Erwan Gouriou <erwan.gouriou@linaro.org>
7 files changed