Enforce DEFAULTTUNE even if overrides are used

Enforce DEFAULTTUNE even if overrides are used

Some BSPs do use a series of overrides. This can take over DEFAULTTUNE
which we try to set for all boards in the same way.

Bug-AGL: SPEC-5014
Change-Id: Iffd9c9cc4bf3352eea1c8b1a030202ac76ee1179
Signed-off-by: Jan-Simon Moeller <jsmoeller@linuxfoundation.org>
Reviewed-on: https://gerrit.automotivelinux.org/gerrit/c/AGL/meta-agl/+/29530
Tested-by: Jenkins Job builder account
ci-image-build: Jenkins Job builder account
ci-image-boot-test: Jenkins Job builder account