scripts/oe-setup-layers: write a list of layer paths into the checkout's top dir

scripts/oe-setup-layers: write a list of layer paths into the checkout's top dir

This is beneficial for setting up builds, as this list can be used
to determine reliably where the actual layers are, and discover
available configurations from them.

Also adjust the selftest to check the presence of that file rather
than any specific layer in a hardcoded location.

Sample output (paths are written relative to the file for relocatability
and ease of reading):

{
    "layers": [
        "meta-openembedded/meta-filesystems",
        "meta-openembedded/meta-gnome",
        "meta-openembedded/meta-initramfs",
        "meta-openembedded/meta-multimedia",
        "meta-openembedded/meta-networking",
        "meta-openembedded/meta-oe",
        "meta-openembedded/meta-perl",
        "meta-openembedded/meta-python",
        "meta-openembedded/meta-webserver",
        "meta-openembedded/meta-xfce",
        "poky/bitbake/lib/layerindexlib/tests/testdata/layer1",
        "poky/bitbake/lib/layerindexlib/tests/testdata/layer2",
        "poky/bitbake/lib/layerindexlib/tests/testdata/layer3",
        "poky/bitbake/lib/layerindexlib/tests/testdata/layer4",
        "poky/meta-poky",
        "poky/meta-selftest",
        "poky/meta-skeleton",
        "poky/meta-yocto-bsp",
        "poky/meta"
    ],
    "version": "1.0"
}

Signed-off-by: Alexander Kanavin <alex@linutronix.de>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>