commands: keystore: use correct type for s_len ...

Fixes the follwing warning: "passing argument 2 of 'read_file_2'
from incompatible pointer type".

Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>

commands: implement keystore command ...

The keystore command provides access to the barebox keystore.

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