libsel4camkes: Move ELF section externs to sources

Instead of leaving the ELF section extern declarations in header files
where it could be redefined each time it's included, we move the
declarations to the .c files.

trivial: style changed files in libsel4camkes

The style checker was only run over the files that were changed in the
refactoring of CAmkES connector template functionality into
libsel4camkes.

libsel4camkes: Add camkes_dataport_flush_cache()

This is commit is to prepare for the removal of common dataport flushing
functionality from the seL4DTBHardware and seL4HardwareMMIO templates.

libsel4camkes: Add dataport section definitions

This commit adds definitions for the _dataport_frames ELF section.

camkes/templates: expose dataport frame details

Adds a symbol array for hardware MMIOs such that connectees can
find caps and details of each mapped frame.

Fix licenses

Refactored parser init commit.

Wrap dataport array typedefs in structs to reduce surprises in user code.

Both the `Buf` typedef and the internal area defined for a dataport were byte
arrays. When a user was not consciously aware of these being arrays, it was
possible for them perform casts and pointer operations that had surprising
outcomes. This commit makes both of these structs, and then referenced by
pointers. The in-memory data layout should be unaffected, but this should
reduce surprise misbehaviours in user code.

Closes JIRA CAMKES-369

Release snapshot