core-image-tiny-initramfs: Avoid parsing failures

This variable is otherwise unset leading to missing dependency warnings. Give
it the same default as used elsewhere in other recipes.

(From OE-Core rev: 0a1265f4dd90ad9524005ce91b3d0564ce2215e5)

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

meta/recipes-core/images/core-image-tiny-initramfs.bb

@@ -5,6 +5,8 @@ first 'init' program more efficiently. core-image-tiny-initramfs doesn't \
 actually generate an image but rather generates boot and rootfs artifacts \
 that can subsequently be picked up by external image generation tools such as wic."
 
+VIRTUAL-RUNTIME_dev_manager ?= "busybox-mdev"
+
 PACKAGE_INSTALL = "initramfs-live-boot-tiny packagegroup-core-boot dropbear ${VIRTUAL-RUNTIME_base-utils} ${VIRTUAL-RUNTIME_dev_manager} base-passwd ${ROOTFS_BOOTSTRAP_INSTALL}"
 
 # Do not pollute the initrd image with rootfs features