[nvram2env] split implementation into generic & MIPS-based code

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