package/sudo: new config to add sudo group and rule

Signed-off-by: Stephan Henningsen <stephan+buildroot@asklandd.dk>
[yann.morin.1998@free.fr:
  - drop the conditional creation
  - uid -is -'- when username is
  - leading TAB in user table, squeeze other spaces
]
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>