Fix bug in verilator Makefile for 'debug' rule (#513)
Fix bug in verilator Makefile for 'debug' rule (#513)

The 'debug' rule is currently cleaning out the non-debug-model directory
rather than the debug-model directory. This commit fixes that, and
changes both the debug and non-debug rules to use the variables defined
for referring to these two model directories.

Co-authored-by: Tynan McAuley <b0381ff746adb55af39bb6cd8bac21dc896573c0@galois.com>
ucb-bar/chipyard,ucb-bar/chipyard,ucb-bar/chipyard,ucb-bar/chipyard