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/Hummingbird_A31_defconfig b/configs/Hummingbird_A31_defconfig
index 3b0c439..dc4133d 100644
--- a/configs/Hummingbird_A31_defconfig
+++ b/configs/Hummingbird_A31_defconfig
@@ -8,8 +8,8 @@
 CONFIG_VIDEO_VGA_EXTERNAL_DAC_EN="PH25"
 CONFIG_DEFAULT_DEVICE_TREE="sun6i-a31-hummingbird"
 # CONFIG_SYS_MALLOC_CLEAR_ON_INIT is not set
-CONFIG_SPL=y
 CONFIG_SYS_EXTRA_OPTIONS="SUNXI_GMAC,RGMII"
+CONFIG_SPL=y
 # CONFIG_CMD_IMLS is not set
 # CONFIG_CMD_FLASH is not set
 # CONFIG_CMD_FPGA is not set