scripts/patchreview: rework patch detection

scripts/patchreview: rework patch detection

A previous patch[1] added the ability to allow the search pattern for
patches to be changed, so that patchreview can be used across the entire
meta-oe repository by changing the patterns.

However, this means the caller needs to write long patterns when calling
patchreview.

Instead, we can see if the specified directory contains a layer by
checking if conf/layer.conf exists.  If it does, then search for patches
inside this directory.  If it doesn't, assume that the specified
directory is a repository that contains sublayers (such as
meta-openembedded) and look through each of the directories that match
the pattern meta-*.

This means patchreview can both scan either a single layer (eg
.../poky/meta) or a repository of sublayers (eg .../meta-openembedded).

[1] oe-core 599046ea9302af0cf856d3fcd827f6a2be75b7e1

Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>