boards: lpcxpresso55s69: merge dual core images
boards: lpcxpresso55s69: merge dual core images

Merges cpu0 and cpu1 targets to a single image, named multicore.bin,
this image can be found in the build folder.
Documentation is to be updated in a later commit.

Signed-off-by: Andrei Gansari <f1330555a90b7a22d81843f3c4366f58b50eae34@nxp.com>

galak/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,finikorg/zephyr,Vudentz/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,finikorg/zephyr,nashif/zephyr,Vudentz/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,galak/zephyr,Vudentz/zephyr,Vudentz/zephyr,nashif/zephyr,Vudentz/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,nashif/zephyr,nashif/zephyr,galak/zephyr,nashif/zephyr,Vudentz/zephyr