Set PWD to the absolute path of the top tree.

Previously PWD is passed in as a shell environmental variable,
which does not point to top of the source tree if you run mm/mmm.
That in turn breaks mm/mmm if OUT_DIR_COMMON_BASE is set to an absolute
path.

Change-Id: Iac99d5668aee1bbd3cb9897750afc2149bd9f973