docs/manual: add external.desc to list of files needed for BR2_EXTERNAL

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>
(cherry picked from commit a6479d6058b75533dd68081d90cec0dbfe652ca4)
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

docs/manual/customize-directory-structure.txt

@@ -50,6 +50,7 @@ to you.
 |
 +-- Config.in (if using a br2-external tree)
 +-- external.mk (if using a br2-external tree)
++-- external.desc (if using a br2-external tree)
 ------
 
 Details on the files shown above are given further in this chapter.