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/pxm2_defconfig b/configs/pxm2_defconfig
index bafe4f4..79b9cae 100644
--- a/configs/pxm2_defconfig
+++ b/configs/pxm2_defconfig
@@ -2,9 +2,9 @@
 CONFIG_TARGET_PXM2=y
 CONFIG_SYS_MALLOC_F_LEN=0x2000
 CONFIG_DEFAULT_DEVICE_TREE="am335x-pxm50"
-CONFIG_SPL=y
 CONFIG_FIT=y
 CONFIG_BOOTDELAY=3
+CONFIG_SPL=y
 CONFIG_HUSH_PARSER=y
 CONFIG_SYS_PROMPT="U-Boot# "
 CONFIG_AUTOBOOT_KEYED=y