mvebu: unset uboot build by default

Initialise the UBOOT variable by default. Otherwise it will be
unintended inherit to following images if set and causes an uboot build
where not required.

Signed-off-by: Mathias Kresin <dev@kresin.me>

target/linux/mvebu/image/Makefile

@@ -87,7 +87,9 @@ define Device/Default
   KERNEL_NAME := zImage
   KERNEL := kernel-bin | append-dtb | uImage none
   SUPPORTED_DEVICES = $$(DEVICE_DTS)
+  UBOOT :=
 endef
+DEVICE_VARS += UBOOT
 
 define Device/UBI
   IMAGES := sysupgrade.bin

target/linux/mvebu/image/cortex-a9.mk

@@ -158,7 +158,6 @@ define Device/turris-omnia
   IMAGE/omnia-medkit-$$(IMAGE_PREFIX)-initramfs.tar.gz := omnia-medkit-initramfs | gzip
   IMAGE_NAME = $$(2)
   DEVICE_DTS := armada-385-turris-omnia
-  UBOOT :=
 endef
 TARGET_DEVICES += turris-omnia