Make SCons+SLICC more clear

[powerjg]

I'll update the book to be more clear about how to get SCons to pick up the new protocol. The magic is to create a SConsopts file in the same directory as your protocol and add the line "protocol_dirs.append(str(Dir('.').abspath))". This tells the SLICC compiler to look in that directory when finding all of the protocols.

Here: http://learning.gem5.org/book/part3/MSI/cache-intro.html#first-steps-to-writing-a-protocol