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_boneblack_vboot_defconfig b/configs/am335x_boneblack_vboot_defconfig
index addfd09..596a083 100644
--- a/configs/am335x_boneblack_vboot_defconfig
+++ b/configs/am335x_boneblack_vboot_defconfig
@@ -27,6 +27,7 @@
 CONFIG_FASTBOOT_BUF_ADDR=0x81000000
 CONFIG_FASTBOOT_BUF_SIZE=0x07000000
 # CONFIG_CMD_IMLS is not set
+CONFIG_CMD_SPL=y
 # CONFIG_CMD_FLASH is not set
 # CONFIG_CMD_SETEXPR is not set
 CONFIG_OF_CONTROL=y