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/am335x_sl50_defconfig b/configs/am335x_sl50_defconfig
index dae8f1b..66da4d3 100644
--- a/configs/am335x_sl50_defconfig
+++ b/configs/am335x_sl50_defconfig
@@ -1,11 +1,11 @@
 CONFIG_ARM=y
 CONFIG_TARGET_AM335X_SL50=y
 CONFIG_SPL_STACK_R_ADDR=0x82000000
-CONFIG_SPL=y
-CONFIG_SPL_STACK_R=y
 CONFIG_FIT=y
 CONFIG_SYS_EXTRA_OPTIONS="EMMC_BOOT"
 CONFIG_VERSION_VARIABLE=y
+CONFIG_SPL=y
+CONFIG_SPL_STACK_R=y
 CONFIG_HUSH_PARSER=y
 # CONFIG_AUTOBOOT is not set
 CONFIG_CMD_BOOTZ=y