ramips: mt7620: disable Jboot devices build

At this moment kernel size in mt7620 snapshot builds is bigger than 2048k.
It should be disabled by default.

Signed-off-by: Pawel Dembicki <paweldembicki@gmail.com>

target/linux/ramips/image/mt7620.mk

@@ -67,6 +67,7 @@ define Device/amit_jboot
   IMAGE/sysupgrade.bin := mkdlinkfw | pad-rootfs | append-metadata
   IMAGE/factory.bin := mkdlinkfw | pad-rootfs | mkdlinkfw-factory
   DEVICE_PACKAGES := jboot-tools kmod-usb2 kmod-usb-ohci
+  DEFAULT := n
 endef
 
 define Device/asus_rp-n53