pipeline: meson: Allow nested pipeline handler directory structures

The current pipeline handler build files require a flat directory
struture for each pipeline handler. Modify the build files to remove
this restriction and allow a directory structure such as:

src/libcamera/pipeline/
  |- raspberrypi
      |- common
      |- vc4
  |- rkisp1
  |- ipu3

where each subdir (e.g. raspberrypi/common, raspberrypi/vc4) has its own
meson.build file. Such a directory structure will be introduced for the
Raspberry Pi pipeline handler in a future commit.

Signed-off-by: Naushir Patuck <naush@raspberrypi.com>
Reviewed-by: Jacopo Mondi <jacopo.mondi@ideasonboard.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>

src/libcamera/pipeline/meson.build

@@ -3,6 +3,15 @@
 # Location of pipeline specific configuration files
 pipeline_data_dir = libcamera_datadir / 'pipeline'
 
+# Allow multi-level directory structuring for the pipeline handlers if needed.
+subdirs = []
+
 foreach pipeline : pipelines
+    pipeline = pipeline.split('/')[0]
+    if pipeline in subdirs
+        continue
+    endif
+
     subdir(pipeline)
+    subdirs += pipeline
 endforeach