Use SRCDIR as a working directory

The existing behavior of using the build directory as the working
directory is useful if you want to move/copy the output directory around
and SRCDIR still refers the the source. But, it's more useful to have
the source directory be the working directory. Tools like cpp(__FILE__)
and other debug prints embed relative paths from the working directory.
We also have tools that expect the working directory to be $TOP.

Change-Id: Ia0f1d3c6b7df72d61cf5628efa2baa98bd19775b
diff --git a/soong.bootstrap.in b/soong.bootstrap.in
index 79e4a65..be71d15 100644
--- a/soong.bootstrap.in
+++ b/soong.bootstrap.in
@@ -1,2 +1,3 @@
-SRCDIR_IN="@@SrcDir@@"
+BUILDDIR="@@BuildDir@@"
+SRCDIR_FROM_BUILDDIR="@@SrcDirFromBuildDir@@"
 PREBUILTOS="@@PrebuiltOS@@"