services: agetty: Add agetty instance to base services. Make its tty optional.

* gnu/services/base.scm (%base-services): Instantiate agetty-service.
(default-serial-port): New variable.
(agetty-shepherd-service): Make tty optional, default to the above.
* doc/guix.texi (agetty-configuration): Update "tty" documentation.
* gnu/system/install.scm (agetty-default-service): Delete variable.
(embedded-installation-os): Remove agetty-default-service instance.
Add "console" kernel-argument.