shell: move shell to its own subsystem

This moves the shell component into its own subsys and groups all
related files and options into a single place.

Additionally, one Kconfig option will now be required to enable the
shell:

 CONFIG_CONSOLE_SHELL=y

The header files was also moved to include/shell/shell.h and can be now
referenced with

 #include <shell/shell.h>

instead of

 #include <misc/shell.h>

Updated documentation as well.

Change-Id: Iffbba4acfa05408055e9fd28dffa213451351f94
Signed-off-by: Anas Nashif <anas.nashif@intel.com>