Move math function definitions.

Created math.mk and moved all math function definitions there.
Then, included this file in config.mk. This allows the functions to be
used by whatever includes config.mk (envsetup and build).

Test: manually setting combinations of PRODUCT_SHIPPING_API_LEVEL
  and PRODUCT_FULL_TREBLE_OVERRIDE and looking at values of
  PRODUCT_FULL_TREBLE.
Bug: 62229856
Change-Id: Icdab4214a1e65ae202411613dbcb9c9ea5f43b09