Improve performance (don't filter ALL_MODULES)

It turns out that this single line was responsible for 28-44% of the
time we spent parsing makefiles. So instead of filtering across
$(ALL_MODULES), check to see if a variable that we always would set is
set.

Test: treehugger
Test: check kati-build eval time before/after
Change-Id: I946c2efd060181a4ae9edd3409c53d9b49a58285