rsa: Allow to directly compile in rsa public keys ...

So far we relied on the U-Boot mkimage tool to generate us device tree
snippets containing rsa public keys which we then compiled into barebox.
Make this easier and allow to directly specify a filename or PKCS#11 URI
in Kconfig. With this we no longer need the U-Boot mkimage tool here and
no more external steps to prepare device tree snippets.

With this rsa public keys can be directly compiled as C structs into
barebox which is much more direct than putting it into the device tree.

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

rsa: let rsa_of_read_key() return a fully allocated key ...

Until now rsa_of_read_key() took a pointer to a key and filled the
struct rsa_public_key members with allocated values. So far we have
never freed these values. Change rsa_of_read_key() to always return
a fully allocated key and provide rsa_key_free() to free it. Let the
FIT image code free the key after usage.

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

crypto: add RSA support ...

Taken from U-Boot and adopted to barebox with little changes.

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