. define _MINIX_SYSTEM for all system code from minix.service.mk
. hide some system-level declarations and definitions
behind _MINIX_SYSTEM to cleanly fix host tool build problems on
Minix (such as: NONE being defined and paddr_t being used but not
declared)
. the similar definition _SYSTEM is unsuitable as it changes the
values of errno definitions
Change-Id: I407de79e2575115243a074b16e79546a279cfa3e