sdk: copy system-qemu.img to system-images.zip

Since init requires system to have partition table
and the original raw system.img does not have it.
system-qemu.img is now created to contain the exact
raw system.img, plus partition table.

When creating the sdk-repo-{host}-system-images.zip
file, using system-qemu.img instead of system.img.

Change-Id: I68d0b172b86ef232d4facfe806181f2e08d9fdf2