UvisorBoxIndex should wrap all uVisor internal structures

[AlessandroA]

We should wrap the RPC structure inside UvisorBoxIndex to improve the UvisorBssSection abstraction.

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UvisorBssSections contains structures that can be accessed by user-space code. Space for these structs is allocated at runtime in each box private BSS section. The user can then access the sections via dedicated pointers.

At the moment the structure exposes the UvisorBoxIndex struct, the RPC struct, the private context and the private (static) heap. While for the private context and heap it makes sense to expose a separate pointer (we expect the user to access them individually), the RPC struct is for internal use only and should not be exposed.

Note: This is just a UX/abstraction issue. The user-space code can still access the RPC structure.

[ciarmcom]

ARM Internal Ref: IOTSEC-298