configs/qemu*: use the post-image script with "$(BR2_DEFCONFIG)" as argument.

This commit allow to use the post-image script previously added
for each qemu board.

Add "$(BR2_DEFCONFIG)" as post-image script argument to retried
the qemu command line.

Signed-off-by: Romain Naour <romain.naour@smile.fr>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>