configs/qemu_arm_vexpress_tz_defconfig: build start-qemu.sh

configs/qemu_arm_vexpress_tz_defconfig: build start-qemu.sh

When tags was added by commit 011206b2bffd509b29365bc1eaa761414ee13726
to detect the qemu command line, the qemu_arm_vexpress_tz_defconfig
was ignored due to a build issue.

This build issue has been fixed by previous patches, so we can
enable the runtime testing by adding the tag in the readme.txt
and the post-image script in the defconfig.

Since Qemu from HOST_DIR is now executed directly from BINARIES_DIR,
we can remove all the string before "qemu-system-*".

Signed-off-by: Romain Naour <romain.naour@gmail.com>
Cc: Yann E. MORIN <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>