support/scripts/boot-qemu-image.py: don't fail if start-qemu.sh is missing

support/scripts/boot-qemu-image.py: don't fail if start-qemu.sh is missing

When boot-qemu-image.py script was added, we wanted to run
each qemu defconfig in gitlab, so we expect that all qemu
defconfig generate the script start-qemu.sh in images
directory.

Don't make it a hard requirement even if we prefer to be
able to do a runtime test for each qemu defconfig.

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>