So that we don't have to load up all the makefile state just to remove
the output directory.
Starting from a completely empty out directory:
kati: 16s
soong_ui: 2.0s
From a minimal out directory (m -j blueprint_tools):
kati: 3.8s
soong_ui: 0.4s
Test: m -j clean
Test: m -j clobber
Change-Id: Ibeec6fbfa29387750342a752a55336caca4b3992
0b73b4bc37