Make LOG_LENGTH configurable per FPGA variant

Make LOG_LENGTH configurable per FPGA variant

This plumbs the LOG_LENGTH parameter (which controls how many entries
the core log RAM has) up to the top level so that it can be set on
the fusesoc command line and have different default values on
different FPGAs.

It now defaults to 512 entries generally and on the Artix-7 35 parts,
and 2048 on the larger Artix-7 FPGAs.  It can be set to 0 if desired.

Signed-off-by: Paul Mackerras <paulus@ozlabs.org>