make: Added relevant env variables to help text

We already hinted PERFGEN in make perf's help text. This extends the
idea to the other rules that depend on env variables.

The default behavior of erroring because make test/bench was run without
the right env variable is confusing enough.

Makefile

@@ -368,7 +368,7 @@ structs-csv: $(BUILDDIR)/lfs3.structs.csv
 structs-diff: $(OBJ)
 	./scripts/structs.py $^ $(STRUCTSFLAGS) -d $(BUILDDIR)/lfs3.structs.csv
 
-## Find the line/branch coverage after a test run
+## Find line/branch coverage after a test run with COVGEN
 .PHONY: cov
 cov: COVFLAGS+=-s
 cov: $(GCDA)
@@ -387,7 +387,7 @@ cov-diff: $(GCDA)
 		$(patsubst %,-F%,$(SRC)) \
 		$(COVFLAGS) -d $(BUILDDIR)/lfs3.cov.csv)
 
-## Find the perf results after bench run with PERFGEN
+## Find perf results after bench run with PERFGEN
 .PHONY: perf
 perf: PERFFLAGS+=-S
 perf: $(BENCH_PERF)
@@ -406,7 +406,7 @@ perf-diff: $(BENCH_PERF)
 		$(patsubst %,-F%,$(SRC)) \
 		$(PERFFLAGS) -d $(BUILDDIR)/lfs3.perf.csv)
 
-## Find the perfbd results after a bench run
+## Find perfbd results after a bench run with PERFBDGEN
 .PHONY: perfbd
 perfbd: PERFBDFLAGS+=-S
 perfbd: $(BENCH_TRACE)
@@ -527,7 +527,7 @@ testmarks: $(TEST_CSV)
 		-fruntime=test_runtime \
 		$(SUMMARYFLAGS))
 
-## Save the test results
+## Save the test results with TESTMARKS
 .PHONY: testmarks-csv
 testmarks-csv: $(BUILDDIR)/lfs3.test.csv