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/sniper_defconfig b/configs/sniper_defconfig
index b7ded76..deb2261 100644
--- a/configs/sniper_defconfig
+++ b/configs/sniper_defconfig
@@ -11,7 +11,7 @@
 # CONFIG_SYS_MMCSD_RAW_MODE_U_BOOT_USE_SECTOR is not set
 CONFIG_SYS_MMCSD_RAW_MODE_U_BOOT_USE_PARTITION=y
 CONFIG_SYS_MMCSD_RAW_MODE_U_BOOT_PARTITION=2
-# CONFIG_SPL_EXT_SUPPORT is not set
+# CONFIG_SPL_FS_EXT4 is not set
 # CONFIG_SPL_NAND_SUPPORT is not set
 CONFIG_SYS_PROMPT="sniper # "
 # CONFIG_CMD_FLASH is not set