external.desc must be present when using a br2-external tree. The
documentation notes this later in the text, but the file is missing
from the initial overview of files.
Fixes bug #11481.
Signed-off-by: Philipp Wagner <mail@philipp-wagner.com>
Acked-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>