Generalise composite claims

Create a CompositeAttestClaim class for representing claims, that may
contain other claims.

This basically means that the code of SWComponentsClaim is refactored a
bit (mostly renaming) to be more general.

The commit also makes AttestationClaim class abstract, and adds some
documentation to AttestationClaim methods.

Change-Id: I2fe45bf8e540a7ffd41f640aff719bc87444ef57
Signed-off-by: Mate Toth-Pal <mate.toth-pal@arm.com>
4 files changed