specs: describe optional linux-appendroot key

The linux-appendroot key is an extension[1] to the bootloader
specification implemented by barebox since 2015[2].

When set, it instructs barebox to generate a suitable root=
command-line:

  - If global.bootm.root_dev is set to a partition known to barebox,
    barebox will detect the file system on it and generate the suitable
    root command-line argument

  - If global.bootm.root_dev is not set, barebox will use the file
    system containing the bootloader specification entries as the
    Linux rootfs.

The former is useful in A/B setups, where bootloader spec entries are
shipped as part of an immutable partition image, which is at odds with
customizing the root= option according to whether the A or B system is
used.

The latter is especially useful for remote file systems like NFS or 9P:
By placing bootloader specification entries directly into the rootfs,
these file systems can be booted remotely by just pointing barebox at
their remote path.

Describe this key in the specification with the same boolean syntax that
loader.conf is using for its boolean keys.

[1]: End of https://www.barebox.org/doc/latest/user/booting-linux.html#boot-loader-specification
[2]: barebox/barebox@716fdbf

Signed-off-by: Ahmad Fatoum <ahmad@a3f.at>

Author: a3f

specs/boot_loader_specification.md

@@ -285,6 +285,12 @@ The following keys are recognized:
 
   Example: `options root=UUID=6d3376e4-fc93-4509-95ec-a21d68011da2 quiet`
 
+* `linux-appendroot` is a boolean option that, if set to `yes`, instructs the
+  boot loader to generate a suitable `root=` parameter and add it to the
+  Linux kernel command-line prior to any kernel parameters indicated by
+  the `options` key.
+  This key is optional and its absence is treated as `no`.
+
 * `devicetree` refers to the binary device tree to use when executing the
   kernel. This key is optional.
 
@@ -305,6 +311,9 @@ The following keys are recognized:
 
   Example: `architecture aa64`
 
+Boolean arguments may be written as `yes`/`y`/`true`/`t`/`on`/`1`
+or `no`/`n`/`false`/`f`/`off`/`0`. 
+
 Each boot loader menu entry drop-in snippet must include at least a `linux` or an `efi`
 key. Here is an example for a complete drop-in file: