commands: remove struct command pointer from commands ...

This is unused in all commands and thus can be removed.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>

add magicvar command ...

The magicvar command gives an overview about all environment variables
with a special meaning.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>