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.

Makefile.am

@@ -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