Allow repeated doc builds without errors (Autotools)

Sometimes when building with the Autoconf buildsystem,
the avrdude-html/ subdir already exists and therefore
renaming the newly built avrdude/ subdir to avrdude-html/
cannot succeed.

Removing the old avrdude-html/ subdir first fixes that.

src/doc/Makefile.am

@@ -45,6 +45,7 @@ avrdude-html/avrdude.html: $(srcdir)/$(info_TEXINFOS) $(GENERATED_TEXINFOS)
 	 mkdir -p avrdude-html ; \
 	 mv -f *.html avrdude-html ; \
 	else \
+	 rm -rf avrdude-html; \
 	 mv -f avrdude avrdude-html; \
 	fi;