useradd.bbclass: check if a group already exists manually

useradd.bbclass: check if a group already exists manually

The use of groupadd -f makes much more difficult to figure when a
group is not add. This was the case of the class not working for our
usage and this being caused by the lack of '/etc/group' file but
unnoticed as groupadd wasn't failing according.

(From OE-Core rev: 82933a1ff921fd0836f03e6f379fd8536cdc0a30)

Signed-off-by: Otavio Salvador <otavio@ossystems.com.br>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>