shell: Redesign internal command handling
shell: Redesign internal command handling

Make the internal commands (exit, select & help) as any other
commands, so that e.g. "help help" works as expected. Also redesign
the way commands are looked up to avoid duplicate lookups.

Signed-off-by: Johan Hedberg <628991c7b0a19c384f9b99c21ec330ce952bb838@intel.com>

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