improve emulate.sh

• extract kernel and initrd from /boot partition of the system image
• pass system image in as command line argument
• execute with taskset --cpu-list 2,3,4,5 on the a311d to support kvm
• replace shell process with qemu to support running under timeout and other wrappers that want to send a signal to their direct children
• extend sparsedd.py so that it's able to copy with an input offset and optional input byte count