Always cleanup the makedepend backup files

Consistently cleanup the depend.bak files created by makedepend. Also,
realclean has been changed to use a single find command, which speeds it
up considerably.
diff --git a/Makefile b/Makefile
index 62ec070..c69dfce 100644
--- a/Makefile
+++ b/Makefile
@@ -31,10 +31,8 @@
 	-rm -f $(TOP)/configs/current
 	-rm -f $(TOP)/configs/autoconf
 	-rm -rf autom4te.cache
-	-rm -f `find . -name \*.o`
-	-rm -f `find . -name \*.a`
-	-rm -f `find . -name \*.so`
-	-rm -f `find . -name depend`
+	-find . '(' -name '*.o' -o -name '*.a' -o -name '*.so' -o \
+	  -name depend -o -name depend.bak ')' -exec rm -f '{}' ';'