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/am335x_boneblack_vboot_defconfig b/configs/am335x_boneblack_vboot_defconfig
index 2c12402..4d34b88 100644
--- a/configs/am335x_boneblack_vboot_defconfig
+++ b/configs/am335x_boneblack_vboot_defconfig
@@ -2,13 +2,13 @@
 CONFIG_TARGET_AM335X_EVM=y
 CONFIG_SPL_STACK_R_ADDR=0x82000000
 CONFIG_DEFAULT_DEVICE_TREE="am335x-boneblack"
-CONFIG_SPL=y
-CONFIG_SPL_STACK_R=y
 CONFIG_FIT=y
 CONFIG_FIT_VERBOSE=y
 CONFIG_FIT_SIGNATURE=y
 CONFIG_SYS_EXTRA_OPTIONS="EMMC_BOOT,ENABLE_VBOOT"
 CONFIG_VERSION_VARIABLE=y
+CONFIG_SPL=y
+CONFIG_SPL_STACK_R=y
 CONFIG_HUSH_PARSER=y
 CONFIG_AUTOBOOT_KEYED=y
 CONFIG_AUTOBOOT_PROMPT="Press SPACE to abort autoboot in %d seconds\n"