Allow runtime configuration of "zero-delay" check

Define the new "-2" value for bootdelay to mean autoboot with no delay
and don't check for an abort key (while "0" value means do check).

Signed-off-by: Joe Hershberger <joe.hershberger@ni.com>
diff --git a/README b/README
index 0d17e7d..a668748 100644
--- a/README
+++ b/README
@@ -704,6 +704,8 @@
 - Boot Delay:	CONFIG_BOOTDELAY - in seconds
 		Delay before automatically booting the default image;
 		set to -1 to disable autoboot.
+		set to -2 to autoboot with no delay and not check for abort
+		(even when CONFIG_ZERO_BOOTDELAY_CHECK is defined).
 
 		See doc/README.autoboot for these options that
 		work with CONFIG_BOOTDELAY. None are required.