sysmocom-image: Add package-management as IMAGE_FEATURES

In post edison we will need to ask the package class to not throw
away the /var/lib/opkg directory with the state of installed packages.

recipes-apps/images/sysmocom-image.inc

@@ -1,6 +1,7 @@
 DEPENDS = "${MACHINE_EXTRA_RDEPENDS}"
 IMAGE_INSTALL = "task-core-boot ${ROOTFS_PKGMANAGE_BOOTSTRAP} ${ROOTFS_PKGMANAGE} task-osmocom task-sysmocom task-sysmocom-debug task-sysmocom-tools ${MACHINE_EXTRA_RDEPENDS} "
 IMAGE_LINGUAS = " "
+IMAGE_FEATURES += " package-management "
 LICENSE = "MIT"
 
 inherit core-image