keystore: implement forgetting secrets ...

To be able to change secrets add a function to forget secrets.

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

crypto: add simple keystore ...

This patch adds a simple keystore to barebox. The keystore implements a simple
key-value store to hold arbitrary values.

Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>