angstrom-core-tweaks: Turn weaker assignment for EXTRA_IMAGE_FEATURES into a bit stronger

Right now angstrom images dont get the opkg metadata
into images as a result default opkg operations on say
systemd-image return nop. This started to happen after

OE-Core commit b2cc92595b30d96a79f33ea7a7217834c8b6bff7
where bitbake.conf now has EXTRA_IMAGE_FEATURES ??= ""

that will override the angstrom definition which is what we
dont want. So lets make the angstrom define to be ?=

Signed-off-by: Khem Raj <raj.khem@gmail.com>