Update SDK installation path variable name

Update SDK installation path variable name

Upstream YP changed the variable for the SDK installation folder.
in https://git.yoctoproject.org/poky/commit/?h=dunfell&id=67dbe8a1c2b485d0cec174d28107b0ac8ea77d5f

thus adapt our distro configuration.

Bug-AGL: SPEC-4172
Signed-off-by: Jan-Simon Möller <jsmoeller@linuxfoundation.org>
Change-Id: I4e327593fe498fb44caa74e6e361631c3767d4a7
Reviewed-on: https://gerrit.automotivelinux.org/gerrit/c/AGL/meta-agl/+/26972
Tested-by: Jenkins Job builder account
ci-image-build: Jenkins Job builder account
ci-image-boot-test: Jenkins Job builder account