support/scripts/boot-qemu-image.py: increase the timeout by 10

As for the Buildroot testsuite, multiply every emulator timeout by 10
to avoid sporadic failures in elastic runners.

qemu_arm_vexpress_tz_defconfig tested locally with sucess.

Fixes:
https://gitlab.com/buildroot.org/buildroot/-/jobs/1970084046



Signed-off-by: Romain Naour <romain.naour@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>