doc: massive refactor and simplification to migrate to mkdocs

Signed-off-by: Joachim Wiberg <troglobit@gmail.com>

doc/config/tty.md

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+TTYs and Consoles
+=================
+
+**Syntax:** `tty [LVLS] <COND> DEV [BAUD] [noclear] [nowait] [nologin] [TERM]`  
+  `tty [LVLS] <COND> CMD <ARGS> [noclear] [nowait]`  
+  `tty [LVLS] <COND> [notty] [rescue]`
+
+The first variant of this option uses the built-in getty on the given
+TTY device DEV, in the given runlevels.  DEV may be the special keyword
+`@console`, which is expanded from `/sys/class/tty/console/active`,
+useful on embedded systems.
+
+The default baud rate is 0, i.e., keep kernel default.
+
+> The `tty` stanza inherits runlevel, condition (and other feature)
+> parsing from the `service` stanza.  So TTYs can run in one or many
+> runlevels and depend on any condition supported by Finit.  This is
+> useful e.g. to depend on `<pid/elogind>` before starting a TTY.
+
+**Example:**
+
+    tty [12345] /dev/ttyAMA0 115200 noclear vt220
+
+The second `tty` syntax variant is for using an external getty, like
+agetty or the BusyBox getty.
+
+The third variant is for board bringup and the `rescue` boot mode.  No
+device node is required in this variant, the same output that the kernel
+uses is reused for stdio.  If the `rescue` option is omitted, a shell is
+started (`nologin`, `noclear`, and `nowait` are implied), if the rescue
+option is set the bundled `/libexec/finit/sulogin` is started to present
+a bare-bones root login prompt.  If the root (uid:0, gid:0) user does
+not have a password set, no rescue is possible.  For more information,
+see the [Rescue Mode](rescue.md) section.
+
+By default, the first two syntax variants *clear* the TTY and *wait* for
+the user to press enter before starting getty.
+
+**Example:**
+
+    tty [12345] /sbin/getty  -L 115200 /dev/ttyAMA0 vt100
+    tty [12345] /sbin/agetty -L ttyAMA0 115200 vt100 nowait
+
+The `noclear` option disables clearing the TTY after each session.
+Clearing the TTY when a user logs out is usually preferable.
+  
+The `nowait` option disables the `press Enter to activate console`
+message before actually starting the getty program.  On small and
+embedded systems running multiple unused getty wastes both memory
+and CPU cycles, so `wait` is the preferred default.
+
+The `nologin` option disables getty and `/bin/login`, and gives the
+user a root (login) shell on the given TTY `<DEV>` immediately.
+Needless to say, this is a rather insecure option, but can be very
+useful for developer builds, during board bringup, or similar.
+
+Notice the ordering, the `TERM` option to the built-in getty must be
+the last argument.
+
+Embedded systems may want to enable automatic `DEV` by supplying the
+special `@console` device.  This works regardless weather the system
+uses `ttyS0`, `ttyAMA0`, `ttyMXC0`, or anything else.  Finit figures
+it out by querying sysfs: `/sys/class/tty/console/active`.  The speed
+can be omitted to keep the kernel default.
+
+> Most systems get by fine by just using `console`, which will evaluate
+> to `/dev/console`.  If you have to use `@console` to get any output,
+> you may have some issue with your kernel config.
+
+**Example:**
+
+    tty [12345] @console noclear vt220
+
+On really bare bones systems, or for board bringup, Finit can give you a
+shell prompt as soon as bootstrap is done, without opening any device
+node:
+
+    tty [12345789] notty
+
+This should of course not be enabled on production systems.  Because it
+may give a user root access without having to log in.  However, for
+board bringup and system debugging it can come in handy.
+
+One can also use the `service` stanza to start a stand-alone shell:
+
+    service [12345] /bin/sh -l