samples: remove board_shell

This sample is not working on all boards anymore. Adding every single
shell we have to the prj.conf does not work on every board, due to
missing features.

Instead, we now change how shell modules are enabled and make it
possible to enable shell command and modules for any application when
CONFIG_SHELL is enabled.

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