Clarify naming of cores in an SMP system

[jonathanpallant]

Rather than kmain and kmain2, we should have better names for the cores and the code that runs on them.

Perhaps "Boot Core" and "Secondary Core(s)", inline with Arm PSCI and Trusted Firmware?

See https://trustedfirmware-a.readthedocs.io/en/v2.11/components/secure-partition-manager.html#secondary-cores-boot-up