This patch replaces SHA1 by SHA256 in the 'cert_create' tool, so
certificate signatures are generated according to the NSA Suite B
cryptographic algorithm requirements.

Documentation updated accordingly.

Change-Id: I7be79e6b2b62dac8dc78a4f4f5006e37686bccf6

This patch adds a tool that generates all the necessary elements
to establish the chain of trust (CoT) between the images.

The tool reads the binary images and signing keys and outputs the
corresponding certificates that will be used by the target at run
time to verify the authenticity of the images.

Note: the platform port must provide the file platform_oid.h. This
file will define the OIDs of the x509 extensions that will be added
to the certificates in order to establish the CoT.

Change-Id: I2734d6808b964a2107ab3a4805110698066a04be