am335x: sl50: Enable CONFIG_AUTOBOOT_KEYED

On startup the SL50 board halt at U-Boot prompt. Use CONFIG_AUTOBOOT_KEYED
to enable autoboot for this board and define the <SPACE> key to get the
U-Boot prompt.
Signed-off-by: NEnric Balletbo i Serra <enric.balletbo@collabora.com>

configs/am335x_sl50_defconfig

@@ -23,7 +23,10 @@ CONFIG_SPL_I2C_SUPPORT=y
 CONFIG_SPL_OS_BOOT=y
 CONFIG_SPL_POWER_SUPPORT=y
 CONFIG_SPL_YMODEM_SUPPORT=y
-# CONFIG_AUTOBOOT is not set
+CONFIG_AUTOBOOT_KEYED=y
+CONFIG_AUTOBOOT_PROMPT="Press SPACE to abort autoboot in %d seconds\n"
+CONFIG_AUTOBOOT_DELAY_STR="d"
+CONFIG_AUTOBOOT_STOP_STR=" "
 # CONFIG_CMD_IMLS is not set
 CONFIG_CMD_ASKENV=y
 CONFIG_CMD_EEPROM=y