gdbclient.py: set PWD when querying makefiles.

Python's subprocess.check_output() cwd argument does not change the PWD
environmental variable, which is what dumpvar.mk uses to turn relative
paths into absolute paths. This led to incorrect symbol paths when
gdbclient was not run from the root directory.

This CL fixes the bug by manually setting PWD before calling the
makefile. It's possible that dumpvar.mk should be using a different
method to get the current directory, but I was scared to change any
core build logic and this approach seems to work just fine.

Change-Id: Ibe3db0bef6694934c512918f7562029313b1deae

scripts/gdbclient.py

@@ -65,7 +65,12 @@ def dump_var(root, variable):
                  "{}/build/core/config.mk".format(root),
                  "dumpvar-{}".format(variable)]
 
+    # subprocess cwd argument does not change the PWD shell variable, but
+    # dumpvar.mk uses PWD to create an absolute path, so we need to set it.
+    saved_pwd = os.environ['PWD']
+    os.environ['PWD'] = root
     make_output = subprocess.check_output(make_args, cwd=root)
+    os.environ['PWD'] = saved_pwd
     return make_output.splitlines()[0]