Conditional weak assignments behave unexpectedly

["Moessbauer, Felix" <felix.moessbauer@siemens.com>]

Dear developers,

I just stumbled upon the following pattern, which needs further
investigation:

- use the qemuamd64 machine on debian-bullseye
- define a custom WKS_FILE in local.conf

While the qemuamd64.conf file contains the following line:
WKS_FILE:debian-bullseye ?= "sdimage-efi-btrfs"
I expected that I'm able to override that unconditionally in the
local.conf using: WKS_FILE = "foo.wks"

However, the final value of the WKS_FILE variable is "sdimage-efi-
btrfs". In the bitbake documentation, I cannot find any example of
combining weak assignments and overrides. However, the implementation
seems to evaluate the weak assignment on the override first. Then, the
WKS_FILE part from the local conf is evaluated and finally the
WKS_FILE:debian-bullseye overrides what was in WKS_FILE. This suspicion
is also backed by the following test:

local.conf: WKS_FILE:debian-bullseye = "foo.wks"
Now, the final value of WKS_FILE is "foo.wks".

I'm unsure if that has ever been like that and was just unnoticed, or
if the behavior change was introduced in bitbake 2.0.
Anyways, we must not use overrides in combination with weak
assignments. And we also need to fix that on the qemuamd64 machine
config immediately (even if we loose some tests by doing that).
Otherwise it is impossible to override the WKS_FILE of the qemuamd64
machine.

Best regards,
Felix