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/corvus_defconfig b/configs/corvus_defconfig
index 975ac4f..d1d048d 100644
--- a/configs/corvus_defconfig
+++ b/configs/corvus_defconfig
@@ -2,9 +2,9 @@
 CONFIG_ARCH_AT91=y
 CONFIG_TARGET_CORVUS=y
 CONFIG_DEFAULT_DEVICE_TREE="at91sam9g45-corvus"
-CONFIG_SPL=y
 CONFIG_SYS_EXTRA_OPTIONS="AT91SAM9M10G45,MACH_TYPE=2066,SYS_USE_NANDFLASH"
 CONFIG_BOOTDELAY=3
+CONFIG_SPL=y
 CONFIG_HUSH_PARSER=y
 CONFIG_SYS_PROMPT="U-Boot> "
 # CONFIG_CMD_BDI is not set