west: core: add search path argument to require()

Allow the runners to override the default search path, e.g. when
provided through command line. The default is to search for the program
binary on the system PATH.

This is useful when the runner allows to optionally override the search
path of the tool, in the case there are multiple versions or
installations, and not necessarily the tools path are present in the
system PATH environment variable. For example:
`tool = self.require(tool_name, path=args.tool_path_override)`

Signed-off-by: Manuel Argüelles <manuel.arguelles@nxp.com>