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.