message: build: use DIR_FUZZ_SEED_CORPUS if specified for cov_fuzz target
  This commit allows the user to specify the location of the
  fuzz_seed_corpus directory on their machine when running the cov_fuzz
  target. If DIR_FUZZ_SEED_CORPUS is specified, then this will be used.
  Otherwise, qa-assets/fuzz_seed_corpus is assumed to be in the bitcoin
  directory.
diff: @@ -193,6 +193,8 @@ LCOV_FILTER_PATTERN = \
  -p "src/secp256k1" \
  -p "depends"
  +DIR_FUZZ_SEED_CORPUS ?= qa-assets/fuzz_seed_corpus
  +
  $(COV_TOOL_WRAPPER):
  @echo 'exec $(COV_TOOL) "$$@"' > $(COV_TOOL_WRAPPER)
  @chmod +x $(COV_TOOL_WRAPPER)
  @@ -205,7 +207,7 @@ baseline_filtered.info: baseline.info
  $(LCOV) -a $@ $(LCOV_OPTS) -o $@
  fuzz.info: baseline_filtered.info
  - @TIMEOUT=15 test/fuzz/test_runner.py qa-assets/fuzz_seed_corpus -l DEBUG
  + @TIMEOUT=15 test/fuzz/test_runner.py $(DIR_FUZZ_SEED_CORPUS) -l DEBUG
  $(LCOV) -c $(LCOV_OPTS) -d $(abs_builddir)/src --t fuzz-tests -o $@
  $(LCOV) -z $(LCOV_OPTS) -d $(abs_builddir)/src
  