TBB: add build option to save private keys

TBB: add build option to save private keys

This patch adds a boolean build option 'SAVE_KEYS' to indicate the
certificate generation tool that it must save the private keys used
to establish the chain of trust. This option depends on 'CREATE_KEYS'
to be enabled. Default is '0' (do not save).

Because the same filenames are used as outputs to save the keys,
they are no longer a dependency to the cert_tool. This dependency
has been removed from the Makefile.

Documentation updated accordingly.

Change-Id: I67ab1c2b1f8a25793f0de95e8620ce7596a6bc3b