In "I, Too, Sing America," freedom is the big goal. By refusing to buckle under the awful pressures of slavery and oppression, the speaker moves ever closer towards eventual freedom and racial equality. He's looking forward to the day America fulfills her promise of freedom.
There's something to be said for mental freedom, too – the speaker firmly believes that he (and his race) is equal to white Americans. Though he is treated poorly, he knows his value and doesn't allow his mind to be imprisoned. It's all about conviction, here. It's the way to freedom.
The historical compression in this poem – the metaphorical "tomorrow" – makes the idea of freedom much stronger throughout the piece.
The speaker in this poem never views himself as anything other than free.