cros_setup_toolchains: Add a comment, revise clean-up error.

The comment says it all. Using porttree/bintree is more correct here,
but less practical. The comment serves to document this implementation
choice so future developers don't have to spend time trying to fix it.

BUG=brillo:918
TEST=None

Change-Id: Id041bab682f88d5d7fe6d7c8eb488a478290236b
Reviewed-on: https://chromium-review.googlesource.com/272932
Reviewed-by: Gilad Arnold <garnold@chromium.org>
Commit-Queue: Gilad Arnold <garnold@chromium.org>
Tested-by: Gilad Arnold <garnold@chromium.org>
diff --git a/scripts/cros_setup_toolchains.py b/scripts/cros_setup_toolchains.py
index 3551b42..25a2e8b 100644
--- a/scripts/cros_setup_toolchains.py
+++ b/scripts/cros_setup_toolchains.py
@@ -435,9 +435,16 @@
       pkg = GetPortagePackage(target, package)
       current = GetInstalledPackageVersions(pkg)
       desired = GetDesiredPackageVersions(target, package)
+      # NOTE: This refers to installed packages (vartree) rather than the
+      # Portage version (porttree and/or bintree) when determining the current
+      # version. While this isn't the most accurate thing to do, it is probably
+      # a good simple compromise, which should have the desired result of
+      # uninstalling everything but the latest installed version. In
+      # particular, using the bintree (--usebinpkg) requires a non-trivial
+      # binhost sync and is probably more complex than useful.
       desired_num = VersionListToNumeric(target, package, desired, True)
       if not set(desired_num).issubset(current):
-        print('Some packages have been held back, skipping clean!')
+        print('Error detecting stable version for %s, skipping clean!' % pkg)
         return
       unmergemap[pkg] = set(current).difference(desired_num)