matchbox-keyboard: daemonize launch

matchbox-keyboard: daemonize launch

The .shbg extension is not properly supported by the Xsession script
provided by xserver-common.

See discussion here:
https://lists.yoctoproject.org/pipermail/poky/2013-December/009463.html

Bug here:
https://bugzilla.yoctoproject.org/show_bug.cgi?id=5546

Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>