OS_NVMemory subreason to read CMOS reset defaults

Ref: ticket #316.

=> R0 = 5 (read CMOS reset default value)

   R1 = location

<= R2 = value that would be used for that location in the event of a CMOS reset

Not sure it’s worth a block read equivalent, since it’s not accessing the actual hardware we’re not getting any performance boost from a single address phase over IIC.

This needs resolving before 5.20, since at that point another ‘Hook’ directory gets spawned inside !Boot, so there isn’t another (clean) opportunity until 5.30 to sort this out.

Sounds OK to me.