yocto-check-layer: show the distro used

yocto-check-layer: show the distro used

It is good to know which distribution is being used, because it is not the same
whether we are using the poky or openembedded-core+bitbake repository.

The default distribution of bitbake is the nodistro while the other is poky.
The nodistro and poky produce different results when used to check the compatibility
of a layer so it is important to inform about the distro in use.

Signed-off-by: Jose Quaresma <jose.quaresma@oss.qualcomm.com>
Signed-off-by: Mathieu Dubois-Briand <mathieu.dubois-briand@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>