Using a pluggable architecture, allow updates delivered via the Update
class to be verified as signed by a certificate. By using plugins, avoid
pulling either axTLS or BearSSL into normal builds.
A signature is appended to a binary image, followed by the size of the
signature as a 32-bit int. The updater takes a verification function
and checks this signature using whatever method it chooses, and if it
fails the update is not applied.
A SHA256 hash class is presently implemented for the signing hash (since
MD5 is a busted algorithm).
A BearSSLPublicKey based verifier is implemented for RSA keys. The
application only needs the Public Key, while to sign you can use
OpenSSL and your private key (which should never leave your control
or be deployed on any endpoints).
An example using automatic signing is included.
Update the docs to show the signing steps and how to use it in the
automatic and manual modes.
Also remove one debugging line from the signing tool.
Saves ~600 bytes when in debug mode by moving strings to PMEM
Windows can't run the signing script, nor does it normally have OpenSSL
installed. When trying to build an automatically signed binary, warn
and don't run the python.
