[PATCH v2] linux-gen: event: add event validation support

[MatiasElo]

Add support for runtime event validation (initially buffer endmark checking). Event validation can be enabled during configure with '--enable-event-validation' [warn/abort] or with --enabled-debug=full.

When event validation is enabled, endmarks are checked in:

• odp_buffer_free() / odp_buffer_free_multi()
• odp_buffer_is_valid()
• odp_event_free() / odp_event_free_multi() / odp_event_free_sp()
• odp_event_is_valid()
• odp_packet_free() / odp_packet_free_multi() /odp_packet_free_sp()
• odp_packet_is_valid()

[JereLeppanen]

I'm not a huge fan of the word myself, but should we use "canary" instead of "endmark"? https://en.wikipedia.org/wiki/Buffer_overflow_protection#Canaries

[MatiasElo]

V2:

• Added README chapter
• Use _odp_snprint() helper
• Fix GCC-7 LTO build failures