[dmesg] Add an in-memory ring buffer console

Add a trivial ring buffer console that can be used to extract the most
recent 8kB of (non-UI) console output as the ${dmesg} setting.

This allows previous console output to be displayed after the screen
has been cleared, such as when a background picture has been loaded.
For example:

    #!ipxe
    console -p http://boot.ipxe.org/ipxe.png
    show -q dmesg

It also allows console output to be captured and sent as part of an
HTTP POST, to allow for remote diagnostics.  For example:

    #!ipxe
    params
    param dmesg ${dmesg:base64}
    imgfetch http://192.168.0.1/api/diags##params

The recorded console output may be cleared if necessary by clearing
the setting:

    clear builtin/dmesg

The name ${dmesg} is chosen as being unlikely to collide with any
existing variables used in end-user scripts.  A separate "dmesg"
command is not provided, but could easily be added if useful.

Note that iPXE supports recursive variable expansion in shell
commands.  Typing an interactive command such as "echo ${dmesg}" or
"param dmesg ${dmesg}" is therefore a great way to exercise the memory
allocator to the point of exhaustion.  Use "show -q dmesg" to show the
ring buffer contents.

Signed-off-by: Michael Brown <mcb30@ipxe.org>