I removed inadvertently a file /usr/lib/libgtkxxxxx which I symlinked to /media/mmc2 to save primary memory space.

Now the device stops booting after startup which is probably due to the fact the system tries to load that library while the /media/mmc2 is not mounted yet.
Is there a command line startup mode (or another way to access the primary file system) so I can restore that lib to the proper directory.
I won't like to reflash the device.

Your only option is booting from mmc, and you'll need to prepare your external mmc in your linux PC for that, and you'll need a bootable rootfs image. You can find over here debian, mer, and ubuntu, if you don't have a diablo rootfs image ready, which you could also prepare from a FIASCO image.