BinhostService: Refactor PrepareBinhostUploads and RegenBuildCache

BUG=chromium:963215, chromium:963211, b:132732435, b:132732743
TEST=run_tests

Change-Id: I708f7f3dc13b6e3f3ec2e861490aad1400331421
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/chromite/+/1640829
Reviewed-by: Alex Klein <saklein@chromium.org>
Tested-by: Alex Klein <saklein@chromium.org>
diff --git a/lib/build_target_util.py b/lib/build_target_util.py
index c418263..d9f91d2 100644
--- a/lib/build_target_util.py
+++ b/lib/build_target_util.py
@@ -41,6 +41,9 @@
     else:
       self.root = GetDefaultSysrootPath(self.name)
 
+  def __str__(self):
+    return self.name
+
 
 def GetDefaultSysrootPath(target_name):
   if target_name: