libcamera: pipeline: Add a platform configuration file helper

Add a new helper function PipelineHandler::configurationFile() that returns
the full path of a named configuration file. This configuration file may be read
by pipeline handlers for platform specific configuration parameters on
initialisation.

The mechanism for searching for the configuration file is similar to the IPA
configuration file:

- In the source tree if libcamera is not installed
- Otherwise in standard system locations (etc and share directories).

When stored in the source tree, configuration files shall be located in a 'data'
subdirectory of their respective pipeline handler directory.

Signed-off-by: Naushir Patuck <naush@raspberrypi.com>
Reviewed-by: David Plowman <david.plowman@raspberrypi.com>
Reviewed-by: Kieran Bingham <kieran.bingham@ideasonboard.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Kieran Bingham <kieran.bingham@ideasonboard.com>

include/libcamera/internal/pipeline_handler.h

@@ -65,6 +65,9 @@ public:
 	bool completeBuffer(Request *request, FrameBuffer *buffer);
 	void completeRequest(Request *request);
 
+	std::string configurationFile(const std::string &subdir,
+				      const std::string &name) const;
+
 	const char *name() const { return name_; }
 
 protected: