docs(arm): update docs to reflect rotpk key changes

The hashing algorithm for the rotpk is now HASH_ALG,
not always sha-256. The public development keys are
no longer in the repository and are now generated at
run-time, updates the documentation to reflect this.

Change-Id: Ic336f7aca858e9b6a1af6d6e6dc5f4aa428da179
Signed-off-by: Ryan Everett <ryan.everett@arm.com>
3 files changed