packagegroup-core-basic: set the default login manager

packagegroup-core-basic: set the default login manager

Set the default login manager to 'busybox', drop the mingetty in
the RDEPENDS, use ${VIRTUAL-RUNTIME_login_manager} instead.

mingetty doesn't work with serial consoles, so if the login console
is ttyS0 for example, we get error messages on screen and cannot login
on ttyS0.

The login manager, no matter it's tinylogin or busybox, provides
getty, so we can just rdepend on it.

[YOCTO #4207]

Signed-off-by: Chen Qi <Qi.Chen@windriver.com>
Signed-off-by: Saul Wold <sgw@linux.intel.com>