To copy the screen (framebuffer) execute on i.MX 31 PDK terminal:
dd if=/dev/fb0 of=screen.raw bs=1280 count=480
Where: the value 1280 means 640 * 2 bytes (16bpp) and the value 480 is equal the screen width.
Copy this raw file to your Linux host and execute it to convert to png image:
fbgrab -f screen.raw -w 480 -h 640 -b 16 screen.png
To install fbgrab on Debian/Ubuntu execute: apt-get install fbgrab
