Kconfig: Move SPL settings into their own file

Move the SPL settings into common/spl where most of the SPL code is kept.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/configs/k2g_evm_defconfig b/configs/k2g_evm_defconfig
index 9890ec6..d8cf7d9 100644
--- a/configs/k2g_evm_defconfig
+++ b/configs/k2g_evm_defconfig
@@ -2,9 +2,9 @@
 CONFIG_ARCH_KEYSTONE=y
 CONFIG_TARGET_K2G_EVM=y
 CONFIG_DEFAULT_DEVICE_TREE="k2g-evm"
-CONFIG_SPL=y
 CONFIG_OF_BOARD_SETUP=y
 CONFIG_VERSION_VARIABLE=y
+CONFIG_SPL=y
 CONFIG_HUSH_PARSER=y
 CONFIG_CMD_BOOTZ=y
 # CONFIG_CMD_IMLS is not set