Fix #250: support finit.cond=foo for cmdline <boot/foo> conditions

As a real life example, this can be useful for a mfgtest mode for the
device, wherein the bootloader can be set to finit.cond=test_mode or
finit.cond=user_mode and then specific services can start up in test
mode which wouldn't normally start.

Or even custom firmware update mode such as finit.cond=fw_update_mode
which performs some specific firmware update steps and doesn't launch
normal processes.

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

doc/cmdline.md

@@ -12,6 +12,17 @@ The `bool` setting is one of `on, off, true false, 1, 0`.
 * `debug`: Enable kernel debug.  Debug messages are printed to the
    console until Finit starts up, unless `loglevel=7` (below) is used.
 
+* `finit.cond=foo[,bar[,baz]]`: set `<boot/foo>` condition, optionally
+  multiple conditions can be set using the same option, separated with a
+  comma.  Alternatively, multiple `foo.cond=arg` can be given.  Each will
+  result in a `<boot/arg>` condition being set to control the rest of the
+  system bootstrap.
+
+  Very useful for selecting different boot modes, e.g. manufacturing test,
+  firmware upgrade, or rescue mode.
+
+  > Note: `<boot/...>` conditions cannot be cleared with `initctl`!
+
 * `finit.debug[=bool]`: Enable finit debug.  This is operated
 	independently of the kernel `debug` setting.  New as of Finit v4.
 

doc/conditions.md

@@ -46,11 +46,16 @@ Triggering
 ----------
 
 Conditions are mainly triggered (asserted) by built-in plugins, e.g.,
-`netlink.so` and `pidfile.so`, see below for built-in conditions.  Finit
-also supports user-defined conditions, sometimes referred to as static
-or one-shot conditions.  They live in the `<usr/...` namespace and are
-constrained to a flat hierarchy without any sub-directories, unlike the
-pidfile plugin, for instance.
+
+  - `netlink.so`: provides `<net/...>`
+  - `pidfile.so`: provides `<pid/...>`
+  - Cmdline `finit.cond=arg`: provides `<boot/arg`
+
+See below for built-in conditions.  Finit also supports user-defined
+conditions, sometimes referred to as static or one-shot conditions.
+They live in the `<usr/...` namespace and are constrained to a flat
+hierarchy without any sub-directories, unlike the pidfile plugin, for
+instance.
 
 User-defined conditions are controlled using the `initctl cond set` and
 `initctl cond clear` commands:
@@ -118,6 +123,7 @@ Built-in conditions:
 - `sys/pwr/fail`
 - `sys/key/ctrlaltdel`
 - `usr/foo`
+- `boot/arg`
 
 **Note:** `up` means administratively up, the interface flag `IFF_UP`.
   `running` is the `IFF_RUNNING` flag, meaning operatively up.  The