board/qemu: add defconfig file name as tag after the qemu command line

board/qemu: add defconfig file name as tag after the qemu command line

This commit add the name of the Qemu defconfig file after each
qemu command line in order to retrieve it easily.

Since a readme.txt can be shared between several Qemu defconfig, we
need at least one qemu command line in readme.txt for each defconfig.

For now, ignore the qemu_arm_vexpress_tz_defconfig since it fail to build
due to python script issue [1]. Anyway the arm vexpress boot is tested
with qemu_arm_vexpress_defconfig.

[1] http://lists.busybox.net/pipermail/buildroot/2020-February/273738.html

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