build.pl: add a warning about the cost of: make tail_build.clean

solenv/bin/build.pl

@@ -1514,7 +1514,11 @@ sub cancel_build {
     print STDERR "-----------------------------------------------------------------------\n";
     print STDERR "To rebuild a specific module:\n";
     print STDERR "\n";
-    print STDERR "$ENV{GNUMAKE} $module.clean #optional\n";
+    if ($module eq 'tail_build') {
+	print STDERR "$ENV{GNUMAKE} $module.clean # not recommended, this will re-build almost everything\n";
+    } else {
+	print STDERR "$ENV{GNUMAKE} $module.clean # optional\n";
+    }
     print STDERR "$ENV{GNUMAKE} $module\n";
     print STDERR "\n";
     print STDERR "when the problem is isolated and fixed, re-run '$ENV{GNUMAKE}'\n";

tail_build/README

@@ -15,3 +15,7 @@ To migrate a module <foo> under tail_build, one should:
 * remove the module <foo> from postprocess/prj/build.lst dependencies
 * remove the module <foo> from tail_build/prj/build.lst dependencies
 * add the module name in Module_tail_build.mk at the root of bootstrap
+
+This module of course, takes a lot of thought, and dependency work /
+statting until it has finished it's business, knows exactly what to
+re-build, and can start building just that.