Initial ETM cleanups. Most of these are cosmetic:

Initial ETM cleanups.  Most of these are cosmetic:

 - Add a header comment
 - Line up the ETM context struct, pack it a bit
 - Remove unused context_id (this doesn't support ETMv2 yet)
 - Make most functions static
 - Remove unused string table and other needless lines of code
 - Correct "tracemode" helptext

Also provide and use an etm_reg_lookup() to find entries in the ETM
register cache.  This will help cope with corrected contents of that
cache, which doesn't include entires for non-existent registers.

git-svn-id: svn://svn.berlios.de/openocd/trunk@2750 b42882b7-edfa-0310-969c-e2dbd0fdcd60