Split implementation of nvram2env to generic (MI) & MIPS-based code:
- removed includes like "*siba*", because they are unused
- added nvram2env_mips.c file with MIPS-specific code, code moved from nvram2env.c
- added header file to shared defines/structures/function prototypes between MI and MIPS code
Also this fix allows to implement own nvram2env drivers.
Reviewed by: ray, adrian (mentor)
Approved by: adrian (mentor)
Differential Revision: https://reviews.freebsd.org/D6513
