* Make trigger types writable.

This gives a hardware more options when implementing triggers. Instead
of requiring separate triggers for separate types, they could opt to
reuse the configuration storage.

* Unimplemented tinfo traps, doesn't read 0.

* Update detection algorithm for previous commit

* Explain priority assignment rationale.

Issue #276.

* Make cause priorities suggested.

This fixes a corner case when chaining triggers, #275.

* Reading tselect doesn't guarantee a valid trigger.

* Clarify algorithm to enumerate triggers.

* Reword debuggers reading back tselect.

resethaltreq: Proposal for forcing a hart into debug mode out of reset

resethaltreq: add a bit to dmstatus to indicate whether this functionality is present

resethaltreq: Modify the proposal to have set/clear bits

resethaltreq: correct some usages of debugger->Debug Module

Issue #264.

mprven: Add a bit to enable MPRV to take effect in debug mode

MPRVEN: minor text edits

Appendix: correct & clarify what debugger/DM do

Remove merge commits from the changelog

* Debuggers must not write sbcs while sbbusy is set

Issue #258

* Grammar.

Explain that 1 in hart array mask means selected

Issue #261

* Examples: Give an example of CSR access with Quick Access

* Correct tense in QA CSR example

* QA CSR Example: save to arg0 not arg2

I believe this is what we want for all these cases.

* Overview Diagram: increase number of Progbuf words

* Conver dia to eps/pdf

Clarify triggers

Remove implemented features from Future Ideas.

NMI Clarification

Clarify SBA unsupport access size error.

Maybe solves #252.