@Entity
public class Usuario implements UserDetails {
  private static final long serialVersionUID = 1L;

  @Id
  private String username;

  private String password;
  private String nome;

  @OneToMany(fetch=FetchType.EAGER)
  private List<Role> roles = new ArrayList<Role>();

...
}

@Entity
public class Role implements GrantedAuthority {
  private static final long serialVersionUID = 1L;

  @Id
  private String role;
  private String name;

...
}

Hibernate: create table Usuario (username varchar(255) not null, nome varchar(255), password varchar(255), primary key (username))
Hibernate: create table Usuario_Role (Usuario_username varchar(255) not null, roles_role varchar(255) not null)
Hibernate: alter table Usuario_Role add constraint UK_9ljdlf4fugq6jh14x7obwpi37 unique (roles_role)
Hibernate: alter table Usuario_Role add constraint FKb32xr1fddmr4pxdxuj5u14f56 foreign key (roles_role) references Role (role)
Hibernate: alter table Usuario_Role add constraint FKm7abqk7krlrd3bb61ecux2fnx foreign key (Usuario_username) references Usuario (username)

mysql> show tables;
+-----------------+
| Tables_in_kmcdb |
+-----------------+
| atendimento     |
| role            |
| usuario         |
| usuario_role    |
+-----------------+
4 rows in set (0.00 sec)

mysql> select * from role;
+------------+---------------+
| role       | name          |
+------------+---------------+
| ROLE_ADMIN | Administrador |
| ROLE_SUP   | Supervisor    |
| ROLE_TEC   | Técnico      |
+------------+---------------+
3 rows in set (0.00 sec)

mysql> insert into usuario(username, nome, password) value('admin','admin','admin');
Query OK, 1 row affected (0.03 sec)

mysql> insert into usuario(username, nome, password) value('fillipe','fillipe','fillipe');
Query OK, 1 row affected (0.00 sec)

mysql> insert into usuario_role(usuario_username, roles_role) value('admin','ROLE_ADMIN');
Query OK, 1 row affected (0.05 sec)

mysql> insert into usuario_role(usuario_username, roles_role) value('fillipe','ROLE_SUP');
Query OK, 1 row affected (0.00 sec)

mysql> insert into usuario_role(usuario_username, roles_role) value('fillipe','ROLE_TEC');
Query OK, 1 row affected (0.00 sec)

mysql> select * from usuario_role;
+------------------+------------+
| Usuario_username | roles_role |
+------------------+------------+
| admin            | ROLE_ADMIN |
| fillipe          | ROLE_SUP   |
| fillipe          | ROLE_TEC   |
+------------------+------------+
3 rows in set (0.00 sec)

mysql> insert into usuario_role(usuario_username, roles_role) value('fillipe','ROLE_ADMIN');
ERROR 1062 (23000): Duplicate entry 'ROLE_ADMIN' for key 'UK_9ljdlf4fugq6jh14x7obwpi37'
mysql> insert into usuario_role(usuario_username, roles_role) value('admin','ROLE_SUP');
ERROR 1062 (23000): Duplicate entry 'ROLE_SUP' for key 'UK_9ljdlf4fugq6jh14x7obwpi37'