Do not update previous_XXX.mk when there is no change

I'm adding a rule which regenerates ninja files to kati:

https://github.com/shinh/kati/commit/8666cfba20bf4ff0e59053dcfa7bcac9896bcf16

With this regeneration rule, unnecessary re-generation will happen
when these .mk files are updated even if there are no changes in
their contents. With this patch, these .mk files are updated only
when the contents will be actually changed.

Change-Id: I4c796f9454502f6bb25019b3806ca577ea5258c1