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