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/omap3_logic_defconfig b/configs/omap3_logic_defconfig
index 1eb1ff8..d1eb941 100644
--- a/configs/omap3_logic_defconfig
+++ b/configs/omap3_logic_defconfig
@@ -19,6 +19,7 @@
 CONFIG_SYS_PROMPT="OMAP Logic # "
 # CONFIG_CMD_IMI is not set
 # CONFIG_CMD_IMLS is not set
+CONFIG_CMD_SPL=y
 # CONFIG_CMD_EEPROM is not set
 # CONFIG_CMD_FLASH is not set
 CONFIG_CMD_NAND=y