ser2net: bump to version 2.8

Bump ser2net to version 2.8.

Exclude for nommu (uses fork).
Version 2.7 also did, guess the autobuilders never caught it.

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>