update_kernel_afdo: Fix configuration reading

When the script is running with the absolute path it has to read the
relative path of the config file.
If the script is running for multiple channels reset branch dependent
vars on every channel.

BUG=b:272295936
TEST=tested on chrotomation with command line from cronjob invocation

Change-Id: Iab2c83a1a5773a5c2462e35d8ad12732187bd7fa
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/third_party/toolchain-utils/+/4344954
Reviewed-by: George Burgess <gbiv@chromium.org>
Commit-Queue: Denis Nikitin <denik@chromium.org>
Bot-Commit: Lint 🤖 <android-build-ayeaye@system.gserviceaccount.com>
1 file changed