support/testing/infra/basetest: support br2-external

Some upcoming test cases can use one or more br2-external trees as
fixtures that provide packages used only in runtime tests.

Add support for br2-external into the BRTest class. Any test case can
then provide a list of paths for being used as br2-external trees
during the build of the image to test.

Signed-off-by: Ricardo Martincoski <ricardo.martincoski@datacom.ind.br>
Cc: Arnout Vandecappelle <arnout@mind.be>
Signed-off-by: Matt Weber <matthew.weber@rockwellcollins.com>
Signed-off-by: Daniel J. Leach <dleach@belcan.com>
Signed-off-by: Adam Duskett <Aduskett@gmail.com>
Tested-by: Matthew Weber <matthew.weber@rockwellcollins.com>
[Thomas: use named argument for make_extra_opts.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>