finish build_target_lib.get_default_sysroot_path migration [reland]

This allows us to drop the build_target_lib import from cros_build_lib.

The failing platform2 code that caused this to revert before has been
fixed, and cs/ doesn't show any other latent hits.

BUG=chromium:1170007
TEST=CQ passes

Change-Id: I284d99fc2570eff7c4e843140efdfe5a56acc8d5
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/chromite/+/2814898
Tested-by: Mike Frysinger <vapier@chromium.org>
Auto-Submit: Mike Frysinger <vapier@chromium.org>
Commit-Queue: Sergey Frolov <sfrolov@google.com>
Reviewed-by: Sergey Frolov <sfrolov@google.com>
diff --git a/scripts/cros_workon.py b/scripts/cros_workon.py
index 87fd961..a3fa2ea 100644
--- a/scripts/cros_workon.py
+++ b/scripts/cros_workon.py
@@ -15,6 +15,7 @@
 
 import sys
 
+from chromite.lib import build_target_lib
 from chromite.lib import commandline
 from chromite.lib import cros_build_lib
 from chromite.lib import terminal
@@ -80,7 +81,7 @@
     sysroot = '/'
   elif options.board:
     friendly_name = options.board
-    sysroot = cros_build_lib.GetSysroot(board=options.board)
+    sysroot = build_target_lib.get_default_sysroot_path(options.board)
   else:
     cros_build_lib.Die('You must specify either --host, --board')