stp: take the review notes on types and register reads

stp_enabled is a flag, so say bool. cmpBytes() returns a comparison
result, so say int8_t. The three busy waits this branch adds read the
status straight out of the SFR instead of copying four bytes to xdata
first.

cmd_parser.c

@@ -26,7 +26,7 @@
 #pragma constseg BANK2
 
 extern __code struct machine machine;
-extern __xdata uint8_t stp_enabled;
+extern __xdata bool stp_enabled;
 extern __code uint8_t log_to_phys_port[9];
 
 extern volatile __xdata uint32_t ticks;