check-package: ignore external.mk

check-package: ignore external.mk

The external.mk file in a br2-external usually contains raw makefile
targets. This file is common code and not a package recipe so it should
not be tested against the code-style of a package .mk file.

When using this script to check files in a br2-external tree, usually
the user is responsible for not passing files that check-package do not
understand. But external.mk is special because it is part of the
br2-external structure, so it is likely someone expects it to be
checkable by an in-tree script.

Instead of adding another blob to the manual, just ignore this file.
Only do that when a br2-external is being tested (so with option -b
passed to the script) and also check that it is on the root path of the
br2-external to allow someone to have a package called external.

Reported on bug #11271.

Reported-by: Vitaliy Lotorev <lotorev@gmail.com>
Signed-off-by: Ricardo Martincoski <ricardo.martincoski@gmail.com>
Cc: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
[Arnout: wrap at 80 columns]
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>