Have small bathroom to tile. there is linoleum over concrete. Could I nail hardyboard to the concrete, then tile? If not, what would be best way?

Yes this can be done. Use plywood or the equivilant for vinyl tile. Use sturdy board or "backer board as it is sometimes called) for ceramic tile.
Our company has glued vinyl tile directly to linoleum that was well glued to the floor and had no problem. I know one incidence that this tile has been done this way for over 10 years with no problem. HOWEVER: THIS IS NOT RECOMMENDED

If the linoleum is sound and not coming up you can tile directly using "Tavy Skin" from Tavy Tool Company. It will only add 1/32 in height plus tile and thinset. Use a good quality modified thinset to attached the tile.