Convert CONFIG_SPL_EXT_SUPPORT to Kconfig

Move this option to Kconfig and tidy up existing uses.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/configs/igep0030_defconfig b/configs/igep0030_defconfig
index 9a5eac1..04c8581 100644
--- a/configs/igep0030_defconfig
+++ b/configs/igep0030_defconfig
@@ -4,6 +4,7 @@
 CONFIG_SYS_EXTRA_OPTIONS="MACH_TYPE=MACH_TYPE_IGEP0030,BOOT_ONENAND"
 CONFIG_VERSION_VARIABLE=y
 CONFIG_SPL=y
+# CONFIG_SPL_EXT_SUPPORT is not set
 CONFIG_HUSH_PARSER=y
 CONFIG_CMD_BOOTZ=y
 # CONFIG_CMD_IMLS is not set