doc: application: improvements to "Overriding Default Configuration"
doc: application: improvements to "Overriding Default Configuration"

Make this a bit less Unix-centric, changing the CMake invocation lines
to only use documented parameters that continue to work across all
CMake versions (-B doesn't work everywhere, and -H means "help" in
recent versions of CMake).

Handle some 80-column cleanliness. Add documentation about the search
key, /. Be a bit more explicit about the steps.

Signed-off-by: Marti Bolivar <fd5da480b758a78208111e6ae5c6806e1f887137@opensourcefoundries.com>

Vudentz/zephyr,explora26/zephyr,punitvara/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,explora26/zephyr,explora26/zephyr,aceofall/zephyr-iotos,galak/zephyr,nashif/zephyr,Vudentz/zephyr,nashif/zephyr,galak/zephyr,nashif/zephyr,Vudentz/zephyr,mbolivar/zephyr,Vudentz/zephyr,mbolivar/zephyr,zephyriot/zephyr,aceofall/zephyr-iotos,nashif/zephyr,ldts/zephyr,punitvara/zephyr,explora26/zephyr,Vudentz/zephyr,kraj/zephyr,aceofall/zephyr-iotos,ldts/zephyr,finikorg/zephyr,aceofall/zephyr-iotos,finikorg/zephyr,kraj/zephyr,galak/zephyr,galak/zephyr,kraj/zephyr,GiulianoFranchetto/zephyr,ldts/zephyr,Vudentz/zephyr,finikorg/zephyr,zephyriot/zephyr,punitvara/zephyr,GiulianoFranchetto/zephyr,aceofall/zephyr-iotos,zephyrproject-rtos/zephyr,zephyriot/zephyr,GiulianoFranchetto/zephyr,punitvara/zephyr,GiulianoFranchetto/zephyr,kraj/zephyr,zephyriot/zephyr,ldts/zephyr,zephyriot/zephyr,punitvara/zephyr,zephyrproject-rtos/zephyr,GiulianoFranchetto/zephyr,mbolivar/zephyr,finikorg/zephyr,explora26/zephyr,zephyrproject-rtos/zephyr,nashif/zephyr,mbolivar/zephyr,finikorg/zephyr,galak/zephyr,ldts/zephyr,mbolivar/zephyr,kraj/zephyr