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/doc/README.SPL b/doc/README.SPL
index 48981bd..7a30fef 100644
--- a/doc/README.SPL
+++ b/doc/README.SPL
@@ -54,7 +54,7 @@
 CONFIG_SPL_SPI_FLASH_SUPPORT (drivers/mtd/spi/libspi_flash.o)
 CONFIG_SPL_SPI_SUPPORT (drivers/spi/libspi.o)
 CONFIG_SPL_FS_FAT (fs/fat/libfat.o)
-CONFIG_SPL_EXT_SUPPORT
+CONFIG_SPL_FS_EXT4
 CONFIG_SPL_LIBGENERIC_SUPPORT (lib/libgeneric.o)
 CONFIG_SPL_POWER_SUPPORT (drivers/power/libpower.o)
 CONFIG_SPL_NAND_SUPPORT (drivers/mtd/nand/raw/libnand.o)