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/am3517_evm_defconfig b/configs/am3517_evm_defconfig
index d5a0e30..0f3b04e 100644
--- a/configs/am3517_evm_defconfig
+++ b/configs/am3517_evm_defconfig
@@ -1,11 +1,11 @@
 CONFIG_ARM=y
 CONFIG_OMAP34XX=y
 CONFIG_TARGET_AM3517_EVM=y
-CONFIG_SPL=y
 CONFIG_FIT=y
 CONFIG_SYS_EXTRA_OPTIONS="NAND"
 CONFIG_BOOTDELAY=10
 CONFIG_VERSION_VARIABLE=y
+CONFIG_SPL=y
 CONFIG_HUSH_PARSER=y
 CONFIG_SYS_PROMPT="AM3517_EVM # "
 CONFIG_CMD_BOOTZ=y