Move CMDLENGTH to blocks.h so it can be user configured

[chimps-ahoy]

As far as I can see from looking at the code, the definition of CMDLENGTH to be 50 is completely arbitrary, so we can move this into the configuration file and let the user change it.

if this causes issues for certain values (too high or too low), then that's on the user. For my use, I just set it to 60 and I see no issues.