build: use intermediate lcov coverage file

Otherwise the 'lcov -q --remove' run fails with the following error:
lcov: ERROR: cannot write to coverage.info!

Signed-off-by: Andreas Cadhalpun <Andreas.Cadhalpun@googlemail.com>

Makefile

@@ -177,7 +177,7 @@ clean::
 	$(RM) $(CLEANSUFFIXES)
 	$(RM) $(CLEANSUFFIXES:%=tools/%)
 	$(RM) -r coverage-html
-	$(RM) -rf coverage.info lcov
+	$(RM) -rf coverage.info coverage.info.in lcov
 
 distclean::
 	$(RM) $(DISTCLEANSUFFIXES)

tests/Makefile

@@ -217,8 +217,9 @@ fate-list:
 coverage.info: TAG = LCOV
 coverage.info:
 	$(M)lcov -q -d $(CURDIR) -b $(patsubst src%,./,$(SRC_LINK)) --capture | \
-	    sed "s,$(CURDIR)/\./,$(CURDIR)/," > $@
-	$(M)lcov -q --remove $@ "/usr*" -o $@
+	    sed "s,$(CURDIR)/\./,$(CURDIR)/," > $@.in
+	$(M)lcov -q --remove $@.in "/usr*" > $@
+	$(Q)$(RM) $@.in
 
 lcov:  TAG = GENHTML
 lcov: coverage.info