On startup, the BeagleBone family of boards can occationally halt at
the U-Boot prompt. Due to U-Boot receiving random data on the usart.
Migrate BeagleBone based boards to use CONFIG_AUTOBOOT_KEYED, user
will now have to enter the <SPACE> key to get to U-Boot prompt
Signed-off-by: Robert Nelson <robertcnelson@gmail.com> CC: Craig McQueen <craig.mcqueen@innerrange.com> CC: Tom Rini <trini@konsulko.com> Reviewed-by: Tom Rini <trini@konsulko.com>