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/ds109_defconfig b/configs/ds109_defconfig
index 6d513cf..428ac8c 100644
--- a/configs/ds109_defconfig
+++ b/configs/ds109_defconfig
@@ -22,3 +22,4 @@
 CONFIG_SYS_NS16550=y
 CONFIG_USB=y
 CONFIG_USB_EHCI_HCD=y
+CONFIG_FS_EXT4=y