boards: lpcxpresso55s69: enable TFM with MCUboot

Set building TFM with MCUboot. Set the build configuration to
profile_medium, we need smaller TFM images to fit into flash.
Build MCUboot, TFM, sign it, sign Zephyr NS image and merge all the
images. Also change the other configuration, BL2=OFF, to merge as a
single image.
Update documentation on how to flash the board.

Signed-off-by: Andrei Gansari <andrei.gansari@nxp.com>