Ensure a default value for DEVTEST_PERF_COMMENT

This prevents exiting with an error at the end of devtest.sh if the
variable is not set.

Change-Id: I53ee8cabf8b4f3771f25b0c807d0910f268a90ca
Closes-Bug: #1292555
Merge "Ensure a default value for DEVTEST_PERF_COMMENT"