Use shell sort command

When using builtin sort() function, make will detect changes in
the find sub-expression, and re-parse the whole makefile. Even though file
list after soring is identical. This is bad for incremental builds, as
everytime a python file is modified, the order files that gets returned
by find changes, resulting in long build time.

Test: th
Change-Id: I8cc347b4473a2b241f2f10e016e5968477c482f3