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/at91sam9n12ek_nandflash_defconfig b/configs/at91sam9n12ek_nandflash_defconfig
index af86350..7d87d8e 100644
--- a/configs/at91sam9n12ek_nandflash_defconfig
+++ b/configs/at91sam9n12ek_nandflash_defconfig
@@ -1,9 +1,9 @@
 CONFIG_ARM=y
 CONFIG_ARCH_AT91=y
 CONFIG_TARGET_AT91SAM9N12EK=y
-CONFIG_SPL=y
 CONFIG_SYS_EXTRA_OPTIONS="AT91SAM9N12,SYS_USE_NANDFLASH"
 CONFIG_BOOTDELAY=3
+CONFIG_SPL=y
 CONFIG_HUSH_PARSER=y
 CONFIG_SYS_PROMPT="U-Boot> "
 CONFIG_CMD_BOOTZ=y