Fix OVERRIDES additions after recent DEFAULTTUNE change

Fix OVERRIDES additions after recent DEFAULTTUNE change

Add :agl-devel and :agl-ptest overrides using :append instead of .= operator
to defer its action after the distro config is parsed. The distro config now
resets OVERRIDES list since the recent DEFAULTTUNES change.

Bug-AGL: SPEC-5037

Reported-by: Marius Vlad <marius.vlad@collabora.com>
Change-Id: I82e7832cab421726f783cec80556f59f6f4de312
Signed-off-by: Denys Dmytriyenko <denys@konsulko.com>
Reviewed-on: https://gerrit.automotivelinux.org/gerrit/c/AGL/meta-agl/+/29587
Reviewed-by: Jan-Simon Moeller <jsmoeller@linuxfoundation.org>
Tested-by: Jenkins Job builder account
ci-image-build: Jenkins Job builder account
ci-image-boot-test: Jenkins Job builder account