xhc-hb04 sim configs: remove two restrictions

1) Allow arbitrary threadname (default is "servo-thread").
   To specify non-default thread name use:
     [XHC-HB04_CONFIG]threadname=thename
2) Allow start-pause button connection to arbitrary pin
   but provide means for built-in functionality with:
     [XHC-HB04_CONFIG]start-pause=std_start_pause

minor: consolidate string trims for "{}"
       update README and .txt files