Revert "finish build_target_lib.get_default_sysroot_path migration"

This reverts commit d8d25cdd41261fcc7dcfe1baf7a1b8a965faf183.

Reason for revert: suspected to cause crbug.com/1196818

BUG=chromium:1196818
TEST=CQ+1 and see if this resolves build issues

Original change's description:
> finish build_target_lib.get_default_sysroot_path migration
>
> This allows us to drop the build_target_lib import from cros_build_lib.
>
> BUG=chromium:1170007
> TEST=CQ passes
>
> Change-Id: Icf01d140baa482014acfca800a57db7bb1bdc934
> Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/chromite/+/2810800
> Reviewed-by: Sergey Frolov <sfrolov@google.com>
> Commit-Queue: Mike Frysinger <vapier@chromium.org>
> Tested-by: Mike Frysinger <vapier@chromium.org>

Bug: chromium:1170007
Change-Id: I3bb147420bb75b9076a5725a2005e737412aeed0
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/chromite/+/2811778
Commit-Queue: Jack Rosenthal <jrosenth@chromium.org>
Bot-Commit: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com>
diff --git a/scripts/fwgdb.py b/scripts/fwgdb.py
index f001ee6..6766026 100644
--- a/scripts/fwgdb.py
+++ b/scripts/fwgdb.py
@@ -16,7 +16,6 @@
 
 from elftools.elf.elffile import ELFFile
 
-from chromite.lib import build_target_lib
 from chromite.lib import constants
 from chromite.lib import cros_build_lib
 from chromite.lib import cros_logging as logging
@@ -97,8 +96,7 @@
           (', '.join(unified_build_dirs)))
 
   if not firmware_dir:
-    firmware_dir = os.path.join(
-        build_target_lib.get_default_sysroot_path(board), 'firmware')
+    firmware_dir = os.path.join(cros_build_lib.GetSysroot(board), 'firmware')
 
   # Very old firmware you might still find on GoldenEye had dev.ro.elf.
   basenames = ['dev.elf', 'dev.ro.elf']