Allow u-boot to be silent without forcing Linux to be

That's a bit presumptuous of you, u-boot!

Signed-off-by: Joe Hershberger <joe.hershberger@ni.com>
diff --git a/doc/README.silent b/doc/README.silent
index 70202ce..6d90a0e 100644
--- a/doc/README.silent
+++ b/doc/README.silent
@@ -23,4 +23,6 @@
 
  - When booting a linux kernel, the "bootargs" are fixed up so that
    the argument "console=" will be in the command line, no matter how
-   it was set in "bootargs" before.
+   it was set in "bootargs" before. If you don't want the linux command
+   line to be affected, define CONFIG_SILENT_U_BOOT_ONLY in your board
+   config file as well, and this part of the feature will be disabled.