I just did this and discovered a way.
Like fire tock said, if you have wood with you or other flammable block you can place that block inside the portal on the bottom. I then placed netherrack around it to form a 1x1 hole next to the wood block. But no netherrack next to the wood next to the portal. I then placed lava next to it. Fortunately I had buckets with me, I was going to the nether for some easy lava. Anyways after a couple tries it caught fire and once the block was consumed the residual fire ignited the portal.