spl: Kconfig: Replace CONFIG_SPL_EXT_SUPPORT to CONFIG_SPL_FS_EXT4

Replace CONFIG_SPL_EXT_SUPPORT to CONFIG_SPLY_FS_EXT4 so both
obj-$(CONFIG_$(SPL_)FS_EXT4) and CONFIG_IS_ENABLED(FS_EXT4) can be
used to control the build in both SPL and U-Boot.

Signed-off-by: Tien Fong Chee <tien.fong.chee@intel.com>
Reviewed-by: Tom Rini <trini@konsulko.com>
diff --git a/configs/twister_defconfig b/configs/twister_defconfig
index 5632e6d..04c713f 100644
--- a/configs/twister_defconfig
+++ b/configs/twister_defconfig
@@ -7,7 +7,7 @@
 CONFIG_SPL=y
 CONFIG_NR_DRAM_BANKS=2
 CONFIG_BOOTDELAY=10
-# CONFIG_SPL_EXT_SUPPORT is not set
+# CONFIG_SPL_FS_EXT4 is not set
 CONFIG_SPL_OS_BOOT=y
 CONFIG_HUSH_PARSER=y
 CONFIG_SYS_PROMPT="twister => "