Fix rpi-sdimg after IMAGE_FSTYPE being enforced in poky-agl.conf

Fix rpi-sdimg after IMAGE_FSTYPE being enforced in poky-agl.conf

This changeset fixes the case of the rpi-sdimg which was broken
by the IMAGE_FSTYPE setting introduced in poky-agl.conf (distro-wide).

A new variable AGL_EXTRA_IMAGE_FSTYPES is available now and only used
in AGL (target) images.

Reasoning for this solution:
- any modifications to IMAGE_FSTYPES will also affect things like the initramfs or initrd images.
- we minimized the disk-space requirements by reusing the ext4.xz for the rpi-sdimg

Bug-AGL: SPEC-492

Change-Id: I07fd51dbcf334653cd6033b7ccb663c4d4ed8578
Signed-off-by: Jan-Simon Möller <jsmoeller@linuxfoundation.org>
Reviewed-on: https://gerrit.automotivelinux.org/gerrit/9887
Tested-by: Jenkins Job builder account <agl-jobbuilder@automotivelinux.org>
ci-image-build: Jenkins Job builder account <agl-jobbuilder@automotivelinux.org>