autoboot: add an option to override keyed autoboot

As originally implemented, setting the AUTOBOOT_KEYED config option will
prevent users from breaking into the autoboot script with ctrl-c. Restore
that option with a new config symbol.

Signed-off-by: Mark Langsdorf <mark.langsdorf@calxeda.com>
diff --git a/doc/README.autoboot b/doc/README.autoboot
index ff58a79..14e3660 100644
--- a/doc/README.autoboot
+++ b/doc/README.autoboot
@@ -74,6 +74,7 @@
 	"bootretry" is >= 0.
 
   CONFIG_AUTOBOOT_KEYED
+  CONFIG_AUTOBOOT_KEYED_CTRLC
   CONFIG_AUTOBOOT_PROMPT
   CONFIG_AUTOBOOT_DELAY_STR
   CONFIG_AUTOBOOT_STOP_STR
@@ -135,6 +136,13 @@
 	environment variable you can specify a second, alternate
 	string (which allows you to have two "password" strings).
 
+	The CONFIG_AUTOBOOT_KEYED_CTRLC #define allows for the boot
+	sequence to be interrupted by ctrl-c, in addition to the
+	"bootdelaykey" and "bootstopkey". Setting this variable
+	provides an escape sequence from the limited "password"
+	strings.
+
+
   CONFIG_ZERO_BOOTDELAY_CHECK
 
 	If this option is defined, you can stop the autoboot process