doc: update application docs wrt CONF_FILE
doc: update application docs wrt CONF_FILE

Update the application documentation to discuss the new ability to
use semicolons as separators when there are multiple files. Enabling
semicolon separators also enables CMake lists to manipulate CONF_FILE
from the application side.

Signed-off-by: Michael R Rosen <349891aa28b1154ab545ef035e7fc00049cde134@intel.com>

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