balena-image.inc: Remove += from :append

This fixes the warning:
+= is not a recommended operator combination, please replace it.

Signed-off-by: Florin Sarbu <florin@balena.io>

layers/meta-balena-fsl-arm/recipes-core/images/balena-image.inc

@@ -156,8 +156,8 @@ IMAGE_INSTALL:append:imx8mm-lpddr4-evk = " \
        pciutils \
 "
 
-DEPENDS:append:nitrogen8mm += "imx-boot"
-DEPENDS:append:imx8mm-lpddr4-evk += "imx-boot"
+DEPENDS:append:nitrogen8mm = " imx-boot"
+DEPENDS:append:imx8mm-lpddr4-evk = " imx-boot"
 
 # the BSP sets this variable so let's unset it since this makes our build fail
 unset ROOTFS_SIZE