autoboot: remove CONFIG_ZERO_BOOTDELAY_CHECK

As the help message of CONFIG_BOOTDELAY says, CONFIG_BOOTDELAY=-2
means the autoboot with no delay, with no abort check even if
CONFIG_ZERO_BOOTDELAY_CHECK is defined.

To sum up, the autoboot behaves as follows:

 [1] CONFIG_BOOTDELAY=0 && CONFIG_ZERO_BOOTDELAY_CHECK=y
    autoboot with no delay, but you can abort it by key input

 [2] CONFIG_BOOTDELAY=0 && CONFIG_ZERO_BOOTDELAY_CHECK=n
    autoboot with no delay, with no check for abort

 [3] CONFIG_BOOTDELAY=-1
    disable autoboot

 [4] CONFIG_BOOTDELAY=-2
    autoboot with no delay, with no check for abort

As you notice, [2] and [4] come to the same result, which means we
do not need CONFIG_ZERO_BOOTDELAY_CHECK.  We can control all the
cases only by CONFIG_BOOTDELAY, like this:

 [1] CONFIG_BOOTDELAY=0
    autoboot with no delay, but you can abort it by key input

 [2] CONFIG_BOOTDELAY=-1
    disable autoboot

 [3] CONFIG_BOOTDELAY=-2
    autoboot with no delay, with no check for abort

This commit converts the logic as follow:
  CONFIG_BOOTDELAY=0 && CONFIG_ZERO_BOOTDELAY_CHECK=n
    --> CONFIG_BOOTDELAY=-2

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Reviewed-by: Stefan Roese <sr@denx.de>
Acked-by: Igor Grinberg <grinberg@compulab.co.il>
Reviewed-by: Simon Glass <sjg@chromium.org>
Acked-by: Vladimir Zapolskiy <vz@mleia.com>
Reviewed-by: Heiko Schocher <hs@denx.de>
Acked-by: Christian Riesch <christian.riesch@omicronenergy.com>
Acked-by: Hannes Schmelzer <hannes.schmelzer@br-automation.com>

doc/README.autoboot

@@ -132,14 +132,6 @@ What they do
 	provides an escape sequence from the limited "password"
 	strings.
 
-
-  CONFIG_ZERO_BOOTDELAY_CHECK
-
-	If this option is defined, you can stop the autoboot process
-	by hitting a key even in that case when "bootdelay" has been
-	set to 0. You can set "bootdelay" to a negative value to
-	prevent the check for console input.
-
   CONFIG_RESET_TO_RETRY
 
 	(Only effective when CONFIG_BOOT_RETRY_TIME is also set)