bug: only enable serial-getty@ttyGS0.service when /opt/scripts/boot/am335x_evm.sh is present

fixes: https://github.com/RobertCNelson/omap-image-builder/issues/37

Reported-by: Hank Chan <peng2006@gmail.com>
Signed-off-by: Robert Nelson <robertcnelson@gmail.com>