boards: nucleo_l552ze_q: Leverage consolidated TFM signing code

This commit removes the signing commands from the post build steps,
in order to leverage the consolidated TFM signing code.

Also with the support to adjust the hex base address when signing,
there is no need to run the TFM_UPDATE.sh script. We can use west
flash to flash the merged hex file on the board.

Signed-off-by: Yestin Sun <sunyi0804@gmail.com>