Changes

global_platform/scp: refactor _wrap_cmd_apdu

The _wrap_cmd_apdu methods for SCP02 and SCP03 are a bit hard to read. Let's
refactor them so that it is easier to understand what happens. In particular
that one can not have encryption (cenc) without signing (cmac)

Related: OS#6367
Change-Id: I4c5650337779a4bd1f98673650c6c3cb526d518b