externalsrc: fix duplicate entries in .git/info/exclude

externalsrc: fix duplicate entries in .git/info/exclude

`readlines()` preserves trailing newlines, so the duplicate check
against the stripped link name never matched. Strip lines before
comparing to prevent repeated entries on each devtool modify run.

Signed-off-by: Aditya Kurdunkar <adityakurdunkar2@gmail.com>
Signed-off-by: Mathieu Dubois-Briand <mathieu.dubois-briand@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>