[Buildroot] Using buildroot images (iso/ext2) in real target device (flash drive)

[Willy]

Hi,
I have created my system images (iso and ext2) and I would put one of 
them in the flash drive of the target system.

So far I've used unetbootin to create a bootable system which loaded in 
RAM, now the image is too big for my RAM.

Now I've used dd to put my image in the flash drive:
sudo dd if=rootfs.ext2 of=/dev/sdc1

Then when the machine starts I got the error:

Missing operating system
DISK BOOT FAILURE, PLEASE INSERT...

I "think" I've installed grub on my buildroot image
( package BR2_TARGET_GRUB [=y] ).
How can I test if grub is properly installed and how can I setup it?
Thanks,
-- 
Willy