Added make *-diff rules, quick commands to compare sizes

This required a patch to the --diff flag for the scripts to ignore
a missing file. This enables the useful one liner for making comparisons
with potentially missing previous versions:

    ./scripts/code.py lfs.o -d lfs.o.code.csv -o lfs.o.code.csv

    function (0 added, 0 removed)            old     new    diff
    TOTAL                                  25476   25476      +0

One downside, these previous files are easy to delete as a part of make
clean, which limits their usefulness for comparing configuration
changes...

scripts/coverage.py

@@ -99,14 +99,17 @@ def main(**args):
 
     # find previous results?
     if args.get('diff'):
-        with open(args['diff']) as f:
-            r = csv.DictReader(f)
-            prev_results = [
-                (   result['file'],
-                    result['function'],
-                    int(result['coverage_hits']),
-                    int(result['coverage_count']))
-                for result in r]
+        try:
+            with open(args['diff']) as f:
+                r = csv.DictReader(f)
+                prev_results = [
+                    (   result['file'],
+                        result['function'],
+                        int(result['coverage_hits']),
+                        int(result['coverage_count']))
+                    for result in r]
+        except FileNotFoundError:
+            prev_results = []
 
         prev_total_hits, prev_total_count = 0, 0
         for _, _, hits, count in prev_results: