yocto-check-layer: Allow OE-Core to be tested

yocto-check-layer: Allow OE-Core to be tested

For unknown reasons we've never seemingly run the check layer script
against OE-Core itself. This isn't entirely straightforward as the core
layer is a bit of a special case, we can't for example compare signatures
against ourselve and we can't remove core from bblayers.conf.

Core does have distro, machine and software components too, in the case
of distro, our fallback default settings. Whilst the qemu machines could
be split into a seperate layer directory, core wouldn't then parse at all
standalone due to the lack of any machine so it seems a bit pointless to
do that.

These changes tweak the script to handle core's special cases, specifically
to allow distro and machine directories and to account for the README placed
a directory level higher than other layers.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>