stp: rename stpEnabled and take the comments out of the header

The variable lives in rtlplayground.c, so it is declared in rtl837x_common.h
with the others there, and it follows the naming of the rest.

The header carried comments on the externs that the definitions in
rtl837x_stp.c repeat, sometimes differently, which is one place too many to
keep in step. What only the header had, the value ranges and what the
designated arrays hold, moved to the definitions; the rest is gone. Function
declarations lost their comments too.

The status printer only prints, so its running commentary went. A define
replaces the bare 33 in stp_in(), and the note on the loop check is down to
what applies at that line.

cmd_parser.c

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