refactor(arm): rename ARM_ROTPK_HEADER_LEN

This variable had a misleading name, as it is the length
of the header only when the ROTPK is a hash.
Also rename arm_rotpk_header to match the new pattern.

Change-Id: I36c29998eebf50c356a6ca959ec9223c8837b540
Signed-off-by: Ryan Everett <ryan.everett@arm.com>
5 files changed