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

[mizhka]

Summary

This is simple change to 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.

Test Plan

Tested on BroadCom router RT-N53. Additional testing are welcome.

[mizhka]

• remove directory path from header protector
• rename NVRAM_SIGNATURE to CFE_NVRAM_SIGNATURE

[ray]

Only one comment left.

sys/dev/nvram2env/nvram2env.c

238 ↗

(On Diff #16744)

Think, it's time to remove #if-s.

[mizhka]

• removed "#if 1"s

[ray]

Now, looks fine.

[adrian]

approved!