Change "Bootstrap" so that it will not fail on BK source trees of other versions;

this is important because this 4.0 "Bootstrap" is the one generally used on host "build".

Build-tools/Bootstrap

@@ -288,7 +288,7 @@ unless ($opt_skip_manual)
 		system ("bk cat $opt_docdir/Docs/$file.texi > $target_dir/Docs/$file.texi") == 0
 		or &abort("Could not update $file.texi in $target_dir/Docs/!");
 	}
-        system ("rm $target_dir/Docs/Images/Makefile*") == 0
+        system ("rm -f $target_dir/Docs/Images/Makefile*") == 0
         or &abort("Could not remove Makefiles in $target_dir/Docs/Images/!");
         system ("cp $opt_docdir/Docs/Images/*.* $target_dir/Docs/Images") == 0
         or &abort("Could not copy image files in $target_dir/Docs/Images/!");