package/openrc: drop custom getty service

package/openrc: drop custom getty service

In Buildroot getty is provided by BusyBox or util-linux (agetty). Both
create a /sbin/getty symlink pointing to the actual program, so make the
agetty service use that path instead of /sbin/agetty.

Also start agetty after all other services, to prevent mixing startup
messages with the getty prompt on devices with a single serial console,
as the custom unit we're dopping was doing already.

This allows us to drop the custom getty service, which causes an error
message from openrc 0.42.1 (in a forthcoming patch):

    Error: getty is the name of a real and virtual service.

The patches are required because it's not possible to override the
command or startup order by means of a configuration file. Anyway it's
still better to patch something maintained upstream than depending on a
custom piece that may easily become obsolete.

Signed-off-by: Carlos Santos <unixmania@gmail.com>
[yann.morin.1998@free.fr:
  - expand commit log to explain why we ve 'after *' in the same patch
]
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>