box: move flightrec configuration out of CE repository

We define flight_recorder_cfg struct, box_get_flightrec_cfg function,
and box.internal.cfg_configure_flightrec Lua function in the CE
repository although they are actually needed only in the EE repository.
Let's drop them all from the CE repository and instead define stub
functions box_check_flightrec and box_set_flightrec that would
check/apply box.cfg flight recorder parameters.

While we are at it, add missing comments to flightrec function stubs.

NO_DOC=refactoring
NO_TEST=refactoring
NO_CHANGELOG=refactoring