xvkbd: turn into a generic-package

xvkbd was integrated has an autotools package, but it does not have a
./configure script. And to avoid executing the ./configure, the
xvkbd-2.8-makefile.patch was creating a dummy .stamp_configured stamp
file... Ugly.

So, make xvkbd a normal generic package, with BUILD_CMDS and
INSTALL_TARGET_CMDS, and get rid of the crappy part of the patch.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>