shell: improve shell_execute_cmd function
shell: improve shell_execute_cmd function

The function to execute shell commands: shell_execute_cmd will now
behave similarly to invoking command line commands.
I.e. after the command is executed the command buffer is cleared,
while the cursor is set to the initial position.

Signed-off-by: Jakub Rzeszutko <db01fea5b4491dba4fc272e7096cf665c7b907b9@nordicsemi.no>

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