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>

commands: add i2c commands ...

theses commands allow low level access to i2c bus and can be useful
to setup an i2c device without having to add code, compile and flash
barebox.

Signed-off-by: Eric Bénard <eric@eukrea.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>