Ok, after countless trial and errors, method below seems to work fine. Please note that i had to recompile the buildroot with ffmpeg and fbgrab functionalities.
/home/testApp -platform linuxfb
then
ffmpeg -vcodec rawvideo -f rawvideo -pix_fmt rgb32 -s 720x1280 -i /dev/fb0 -f
image2 -vcodec png test%03d.png