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/axm_defconfig b/configs/axm_defconfig
index 220a49b..93805d4 100644
--- a/configs/axm_defconfig
+++ b/configs/axm_defconfig
@@ -2,9 +2,9 @@
 CONFIG_ARCH_AT91=y
 CONFIG_TARGET_TAURUS=y
 CONFIG_DEFAULT_DEVICE_TREE="at91sam9g20-taurus"
-CONFIG_SPL=y
 CONFIG_SYS_EXTRA_OPTIONS="AT91SAM9G20,MACH_TYPE=2068,BOARD_AXM"
 CONFIG_BOOTDELAY=3
+CONFIG_SPL=y
 CONFIG_HUSH_PARSER=y
 # CONFIG_CMD_BDI is not set
 CONFIG_CMD_BOOTZ=y