cros_build_lib: Move out GetSysroot.

GetSysroot is a utility function for build targets, so
move the implementation to the more specific lib module.

BUG=chromium:1060351
TEST=run_tests

Change-Id: I9b7fbfd28110de38ad08f8f4a736d60e3d61ea93
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/chromite/+/2095747
Commit-Queue: Alex Klein <saklein@chromium.org>
Tested-by: Alex Klein <saklein@chromium.org>
Reviewed-by: Mike Frysinger <vapier@chromium.org>
diff --git a/lib/build_target_lib.py b/lib/build_target_lib.py
index c701c54..c6bb6af 100644
--- a/lib/build_target_lib.py
+++ b/lib/build_target_lib.py
@@ -77,12 +77,11 @@
     return '%s-%s' % (base_command, self.name)
 
 
-def get_default_sysroot_path(build_target_name):
-  """Get the default sysroot path for a build target."""
-  if build_target_name:
-    return os.path.join('/build', build_target_name)
-  else:
-    raise InvalidNameError('Target name is required.')
+def get_default_sysroot_path(build_target_name=None):
+  """Get the default sysroot location or '/' if |build_target_name| is None."""
+  if build_target_name is None:
+    return '/'
+  return os.path.join('/build', build_target_name)
 
 
 def is_valid_name(build_target_name):