util/readline: Add C-u shortcut

util/readline: Add C-u shortcut

Add support for the unix-line-discard readline action, which erases from
the cursor position up to the beginning of the line. The default
binding, C-u, was chosen.

This is useful to quickly erase command input while working on the
monitor interface.

Signed-off-by: Manos Pitsidianakis <manos.pitsidianakis@linaro.org>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-ID: <6772067e1c0d4b1c5310e5446e9e3e1c6b3b5bc0.1718265822.git.manos.pitsidianakis@linaro.org>
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>