[OE-core] Multiconfig builds broken in master

[Alex Kiernan]

I've run into a weird multiconfig problem on master which seems to
have been introduced with

b8730f6 ("bitbake: bitbake: Add support for multiconfig dependencies")

Before that, I could happily use:

bitbake multiconfig:gplv2:production-image

whereas now it fails (gplv2 is a multiconfig which basically just sets
TMPDIR and INCOMPATIBLE_LICENSES).

The error suggests failure to populate IMAGE_FEATURES correctly:

ERROR: Nothing PROVIDES 'production-image'
production-image was skipped: 'ostree' in IMAGE_FEATURES is not a
valid image feature. Valid features: allow-empty-password
allow-root-login autologin dbg-pkgs debug-tweaks dev-pkgs doc doc-pkgs
eclipse-debug empty-root-password hwcodecs muted nfs-client nfs-server
package-management post-install-logging ptest-pkgs read-only-rootfs
splash ssh-server-dropbear ssh-server-openssh staticdev-pkgs
tools-debug tools-profile tools-sdk tools-testapps usb-nmc x11
x11-base x11-sato

(the ostree feature is introduced by a layer derived from the Intel
IoT refkit - https://github.com/intel/intel-iot-refkit/blob/master/meta-refkit-core/classes/ostree-image.bbclass)

If I revert that single commit everything works again.

Clearly there's nothing in that changeset which is touching
IMAGE_FEATURES, so it's something way more subtle!

-- 
Alex Kiernan