R.<x> = GF(2)[]

This works since any element in $\mathbb F_{2^8}$ is fixed by the Frobenius morphism $x\to x^{2^8}$. The field generators are among these elements, we insist that they have the maximal degree eight (of the minimal polynomial over $\mathbb F_2$) to pick the generators among all elements. (All ignored multiplicities mul are equal to one.)