doc: application: fix stale sentence about Kbuild
doc: application: fix stale sentence about Kbuild

The Overview section still mentions Kbuild, but the build system is
CMake now.

Signed-off-by: Marti Bolivar <9d90ac4c7bf6a305f6bfd81a23c7859bc883380e@linaro.org>

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