arm: move CONFIG_PREBOOT="usb start" to KConfig

This commit moves CONFIG_PREBOOT="usb start" to common/KConfig
for all boards also declaring USB_KEYBOARD.

Besides simplifying defconfig files, this also enables support for
board-specific CONFIG_PREBOOT for sunxi boards:
commit 37304aaf60bf ("Convert CONFIG_USE_PREBOOT and CONFIG_PREBOOT to
Kconfig") intended to support CONFIG_PREBOOT, but
include/configs/sunxi-common.h hardcodes preboot as part of internally
defined CONSOLE_STDIN_SETTINGS, silently ignoring any board-specific
CONFIG_PREBOOT.

Signed-off-by: Jonas Smedegaard <dr@jones.dk>
Reviewed-by: Neil Armstrong <narmstrong@baylibre.com>
Series-Cc: Jagan Teki <jagan@amarulasolutions.com>
Series-Cc: Lukasz Majewski <lukma@denx.de>
Series-Cc: Andre Przywara <andre.przywara@arm.com>
diff --git a/common/Kconfig b/common/Kconfig
index 67b3818..62d78c5 100644
--- a/common/Kconfig
+++ b/common/Kconfig
@@ -403,6 +403,7 @@
 
 config USE_PREBOOT
 	bool "Enable preboot"
+	default "usb start" if USB_KEYBOARD
 	help
 	  When this option is enabled, the existence of the environment
 	  variable "preboot" will be checked immediately before starting the