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 37304aaf ("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>