shell: Introduce structured help message

In order to enable better and more consistent help messages for shell
commands, this commit introduces a new SHELL_HELP macro that allows to
document a command's description and usage in a more structured way.

This will allow to get rid of inconsistent (and duplicated!) variations
of "Syntax: ...", "Syntax:\n ...", "Usage: ...", in the commands, and
formatting of the description+usage will be taken care of by the shell
when it detects the help message is using the "structured" format.

Signed-off-by: Benjamin Cabé <benjamin@zephyrproject.org>