As iacdxb mentioned.
Add a corona normal shader to the bump slot, in it, put the corona Bitmap shader and in there, put the normal map you'd like to use but set the color profile to linear and it works perfectly. The normal bump strength is then just changed through the % value of the bump channel, as with any other bump map.
Through the additional bump slot in the cor normal shader you have even more control over the look.
It's a lot easier.