env.py: Add debug_events key to target.config

Add new key "ftrace" to target config. The format is the same as tests'
ftrace key. The only supported key for now is "events" to allow adding
extra events to collect, in addition to the mandatory ones specified in
tests configuration. This is to support investigations of tests
failures.

Co-authored-by: Elieva Pignat <elieva.pignat@arm.com>