Adding RSA verification code to be used by uLoader

This includes the code needed to verify a signed Barebox image,
and a test command that can be run on Barebox (or uLoader, if you
can find the space) to check whether a signed image would be
verified.

The actual verification is not yet integerated into the boot process.

Change-Id: I25ada72df661b518f8904d595689949365ebf93d
11 files changed