kirkwood: clean up FEATURES

Remove dt: it is implicitly enabled based on the kernel config
Remove tar.gz and jffs2_nand: Legacy build code has been removed,
NAND devices are only supported with UBI now.
Remove ubifs: deprecated, use squashfs images instead

Signed-off-by: Felix Fietkau <nbd@nbd.name>

target/linux/kirkwood/Makefile

@@ -9,7 +9,7 @@ include $(TOPDIR)/rules.mk
 ARCH:=arm
 BOARD:=kirkwood
 BOARDNAME:=Marvell Kirkwood
-FEATURES:=targz usb jffs2_nand nand ubifs squashfs dt
+FEATURES:=usb nand squashfs
 CPU_TYPE:=xscale
 MAINTAINER:=Luka Perkov <luka@openwrt.org>