use CONFIG_AUTOBOOT_KEYED to only let autoboot be interrupted by the space character

When connecting a FTDI FT232RL usb to ttl uart adapter to the reform
with TX, RX and GND but not connecting that adapter to another machine
via usb, then the adapter will send garbage signals to the reform during
boot. These will interrupt autoboot and prevent the reform from booting
until either the RX/TX/GND are disconnected or the adapter is connected
via USB to another machine.

To prevent this from happening, do not interrupt autoboot on any key
press but require the space key to be pressed.