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:
  If the source tree does not contain any "Docs/Images/Makefile*" (due to an error,
  or because it is not yet prepared to contain the Images in the source tar-ball),
  a plain "rm" will fail and so cause "Bootstrap" to fail. (This has happened ...)
  Prevent this danger by using "rm -f".

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/!");