target: don't build jffs2 nor images by default

target/linux/adm5120/router_be/target.mk

target/linux/adm5120/router_le/target.mk

target/linux/amazon/Makefile

target/linux/ar7/Makefile

target/linux/at91/Makefile

target/linux/atheros/Makefile

target/linux/au1000/Makefile

target/linux/brcm63xx/Makefile

target/linux/ep93xx/Makefile

target/linux/lantiq/Makefile

target/linux/lantiq/ase/target.mk

target/linux/lantiq/falcon/target.mk

target/linux/lantiq/svip_be/target.mk

target/linux/lantiq/svip_le/target.mk

target/linux/lantiq/xway/target.mk

target/linux/mcs814x/Makefile

target/linux/mvebu/Makefile

target/linux/pxa/Makefile

target/linux/rb532/Makefile

target/linux/sparc/Makefile