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>