qemu: allow to build host variant statically

Compiling Qemu statically allows to use it to chroot into target/. It is a
nice feature, so add an option for it.

Signed-off-by: Jérôme Pouiller <jezz@sysmic.org>
[Thomas: slightly tweak Config.in help text.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>