zephyr: Support RSA, and ECDSA P-256 signing

Make it clear in the top-level Makefile how to configure mcuboot for
Zephyr for a particular signing algorithm.  Currently supported, are the
RSA signatures, and ECDSA with the P-256 curve.  These configuration
lines will select the code built in the bootloader, as well as which
public key gets included with the image.

This also adds a demo public key for the P-256 signatures.
8 files changed