fs: Migrate ext4 to Kconfig

Migrate the following symbols to Kconfig:

CONFIG_FS_EXT4
CONFIG_EXT4_WRITE

The definitions in config_fallbacks.h can now be expressed in Kconfig.

Signed-off-by: Tuomas Tynkkynen <tuomas@tuxera.com>
diff --git a/configs/mx25pdk_defconfig b/configs/mx25pdk_defconfig
index 2905614..563c7ac 100644
--- a/configs/mx25pdk_defconfig
+++ b/configs/mx25pdk_defconfig
@@ -17,5 +17,6 @@
 CONFIG_CMD_FS_GENERIC=y
 CONFIG_DOS_PARTITION=y
 CONFIG_ENV_IS_IN_MMC=y
+CONFIG_FS_EXT4=y
 CONFIG_FS_FAT=y
 CONFIG_OF_LIBFDT=y