shell: Add shell_exec

This adds shell_exec which can be used to execute commands directly
without the use of a console which is useful for both testing as well
as interfacing with applications/upper layer which would like to have
access to shell commands directly.

In addition to that this may be more trivial to interface with instead
of using fifos like uart_register_input and telnet_register_input do.

Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>