autoboot.c: Add feature to stop autobooting via SHA256 encrypted password

This patch adds the feature to only stop the autobooting, and therefor
boot into the U-Boot prompt, when the input string / password matches
a values that is encypted via a SHA256 hash and saved in the environment.

This feature is enabled by defined these config options:
     CONFIG_AUTOBOOT_KEYED
     CONFIG_AUTOBOOT_STOP_STR_SHA256

Signed-off-by: Stefan Roese <sr@denx.de>
Reviewed-by: Simon Glass <sjg@chromium.org>