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/microblaze-generic_defconfig b/configs/microblaze-generic_defconfig
index 92324b4..492be51 100644
--- a/configs/microblaze-generic_defconfig
+++ b/configs/microblaze-generic_defconfig
@@ -21,6 +21,7 @@
 CONFIG_SYS_OS_BASE=0x2c060000
 CONFIG_HUSH_PARSER=y
 CONFIG_SYS_PROMPT="U-Boot-mONStR> "
+CONFIG_CMD_SPL=y
 CONFIG_CMD_ASKENV=y
 CONFIG_CMD_GPIO=y
 CONFIG_CMD_MFSL=y