board/qemu/x86*: use quotes around -append Qemu option value

This helps the toolchains.free-electrons.com build logic re-use
directly the Qemu command lines from this readme.txt file, by having a
-append option similar to the ones used in other readme.txt files.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>