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/cmd/Kconfig b/cmd/Kconfig
index faa8bc6..35fe9d5 100644
--- a/cmd/Kconfig
+++ b/cmd/Kconfig
@@ -1367,17 +1367,20 @@
 
 config CMD_EXT2
 	bool "ext2 command support"
+	select FS_EXT4
 	help
 	  Enables EXT2 FS command
 
 config CMD_EXT4
 	bool "ext4 command support"
+	select FS_EXT4
 	help
 	  Enables EXT4 FS command
 
 config CMD_EXT4_WRITE
 	depends on CMD_EXT4
 	bool "ext4 write command support"
+	select EXT4_WRITE
 	help
 	  Enables EXT4 FS write command