Convert CONFIG_CMD_SPL to Kconfig

This converts the following to Kconfig:
   CONFIG_CMD_SPL

Note that trats does not actually use SPL, so this option can no-longer be
set.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
diff --git a/configs/am335x_sl50_defconfig b/configs/am335x_sl50_defconfig
index 41dd2da..e0956d5 100644
--- a/configs/am335x_sl50_defconfig
+++ b/configs/am335x_sl50_defconfig
@@ -29,6 +29,7 @@
 CONFIG_AUTOBOOT_DELAY_STR="d"
 CONFIG_AUTOBOOT_STOP_STR=" "
 # CONFIG_CMD_IMLS is not set
+CONFIG_CMD_SPL=y
 CONFIG_CMD_ASKENV=y
 CONFIG_CMD_EEPROM=y
 # CONFIG_CMD_FLASH is not set