Lock down console

Two enhancements to lock down uboot to avoid user enter
interactive mode:
1. disable autodelay.
2. ignore ctlr-c completely during boot, so that user can
   not use hold ctrl-c from beginning to interrupt to
   interactive mode.
The second enhancement is simplified from
for shutdown ctrl-c backdoor.

Change-Id: Iacb8dca644cfa86559d9e46e9fa58dcbf673f88e
4 files changed