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/a4m2k_defconfig b/configs/a4m2k_defconfig
index 61f7da7..fcf1ff3 100644
--- a/configs/a4m2k_defconfig
+++ b/configs/a4m2k_defconfig
@@ -1,11 +1,11 @@
 CONFIG_PPC=y
 CONFIG_MPC5xxx=y
 CONFIG_TARGET_A3M071=y
-CONFIG_SPL=y
 CONFIG_FIT=y
 CONFIG_OF_BOARD_SETUP=y
 CONFIG_SYS_EXTRA_OPTIONS="A4M2K"
 CONFIG_BOOTDELAY=3
+CONFIG_SPL=y
 CONFIG_HUSH_PARSER=y
 CONFIG_LOOPW=y
 # CONFIG_CMD_SETEXPR is not set