Merge remote-tracking branch 'aosp/upstream' into master

* aosp/upstream:
  Update documentation
  Unify .ninja_log files
  Define everything in both stages
  Mark intermediates as optional
  Fix and test rebuilds of bootstrap ninja when using tests
  Use microfactory to build the bootstrap minibp
  Add microfactory directly from Soong
  Check for .bootstrap/build.ninja rebuilds too

Bug: 63720725
Test: m -j nothing
Test: mkdir o; ../bootstrap.bash; ./soong
Change-Id: I21ebc6000598ec2347ad742120334bc718af9530