Enforce same IMAGE_FSTYPES across all boards for AGL

Enforce same IMAGE_FSTYPES across all boards for AGL

This changeset enforces the same output images across all boards
to be ext4.xz and tar.xz . No exceptions.

Reasons: disk space, unification of instructions, download size

Bug-AGL: SPEC-492

Change-Id: I41c92ba1d92a6e5854cea470eea6e0b559945d5d
Signed-off-by: Jan-Simon Möller <jsmoeller@linuxfoundation.org>
Reviewed-on: https://gerrit.automotivelinux.org/gerrit/9813
Reviewed-by: Kevin Hilman <khilman@baylibre.com>
Tested-by: Jenkins Job builder account <agl-jobbuilder@automotivelinux.org>
ci-image-build: Jenkins Job builder account <agl-jobbuilder@automotivelinux.org>