Boot time creation of mapped devices
====================================
It is possible to configure a device mapper device to act as the root
device for your system in two ways.

The first is to build an initial ramdisk which boots to a minimal
userspace which configures the device, then pivot_root(8) in to it.

The second is to possible when the device-mapper and any targets are
compiled into the kernel (not a module), one or more device-mappers may
be created and used as the root device at boot time with the parameters
given with the boot line dm=...

Multiple device-mappers can be stacked by specifying the number of
devices. A device can have multiple tables if the the number of tables
is specified.

	<dm>		::= <num-mappers> <device-mapper>+
	<device-mapper>	::= <head> "," <table>+
	<head>		::= <name> <uuid> <mode> [<num-tables>]
	<table>		::= <start> <length> <type> <options> ","
	<mode>		::= "ro" | "rw"
	<uuid>		::= xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx | "none"
	<type>		::= "verity" | "bootcache" | ...

Each tables line may be as normal when using the dmsetup tool except for
two variations:
1. Any use of commas will be interpreted as a newline
2. Quotation marks cannot be escaped and cannot be used without
   terminating the dm= argument.

Unless renamed by udev, the device node created will be dm-0 as the
first minor number for the device-mapper is used during early creation.

The <num-tables> field is optional and assumed to be 1.

Examples
========
An example of booting to a linear array made up of user-mode linux block
devices:

  dm="1 lroot none rw 2, 0 4096 linear 98:16 0, 4096 4096 linear 98:32 0" \
  root=/dev/dm-0

This will boot to a rw dm-linear target of 8192 sectors split across two
block devices identified by their major:minor numbers.  After boot, udev
will rename this target to /dev/mapper/lroot (depending on the rules).
No uuid was assigned.

An example of multiple device-mappers, with the dm="..." contents shown
here split on multiple lines for readability:

  3 vboot none ro,
      0 1768000 bootcache
        device=aa55b119-2a47-8c45-946a-5ac57765011f+1
        signature=76e9be054b15884a9fa85973e9cb274c93afadb6
        cache_start=1768000 max_blocks=100000 size_limit=23 max_trace=20000,
    vroot none ro,
      0 1740800 verity payload=254:0 hashtree=254:0 hashstart=1740800 alg=sha1
        root_hexdigest=76e9be054b15884a9fa85973e9cb274c93afadb6
        salt=5b3549d54d6c7a3837b9b81ed72e49463a64c03680c47835bef94d768e5646fe,
    vram none rw 2,
      0 32768 linear 1:0 0,
      32768 32768 linear 1:1 0,