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/cl-som-am57x_defconfig b/configs/cl-som-am57x_defconfig
index fec5e60..c409d34 100644
--- a/configs/cl-som-am57x_defconfig
+++ b/configs/cl-som-am57x_defconfig
@@ -8,7 +8,7 @@
 CONFIG_NR_DRAM_BANKS=2
 # CONFIG_USE_BOOTCOMMAND is not set
 CONFIG_VERSION_VARIABLE=y
-# CONFIG_SPL_EXT_SUPPORT is not set
+# CONFIG_SPL_FS_EXT4 is not set
 # CONFIG_SPL_NAND_SUPPORT is not set
 CONFIG_SPL_SPI_LOAD=y
 CONFIG_SYS_PROMPT="U-Boot# "