shell: Add a configuration for not printing shell messages

The shell currently prints out `: command not found` and
`Please specify a subcommand.` when invalid commands are
sent.

This can cause issues in certain situations, such as if a U-Boot
console in interactive mode is on the other end of the shell's
UART, where the U-Boot console will stop booting the device
if it receives any characters, and it will print out strings that
the shell then sees as commands. This causes the shell to send
out the messages above, preventing the device running
U-Boot on the other end from booting up.

I added the configurations
`CONFIG_SHELL_MSG_SPECIFY_SUBCOMMAND` and
`CONFIG_SHELL_MSG_CMD_NOT_FOUND` to allow disabling
these messages.

Signed-off-by: Brian Moran <brian.moran@sabantoag.com>