What is the weak*-topology on a set of probability measures?

You can consider the set of probabilities over $\mathcal{F}$ as a subset of the linear space $V(\mathcal{F})$ of all finitely additive (bounded) scalar measures over $\mathcal{F}$ endowed with the variation norm (see Theory of charges (K. Bhaskara Rao, M. Bhaskara Rao), chapter 7). We need to show that this space is the topological dual of another (locally convex) topological vector space.

When $\mathcal{F}$ is a Boolean algebra of subsets of $\Omega$ (which in this case does not have to be a topological space) we define $S(\mathcal{F})$ as the linear space generated by $\{\chi_A:\ A\in\mathcal{F}\}$ (the characteristic functions of the sets in $\mathcal{F}$). $S(\mathcal{F})$ is called the space of simple functions.

Now, for every $f\in S(\mathcal{F})$, $\Vert f\Vert_s:=\sup\vert f\vert<\infty$ so $(S(\mathcal{F}),\Vert\cdot\Vert_s)$ is a normed space. It is not hard to prove that the dual of $(S(\mathcal{F}),\Vert\cdot\Vert_s)$ is the space $V(\mathcal{F})$. In fact, on one hand every $\lambda\in V(\mathcal{F})$ defines the bounded linear functional $f\mapsto \int f\ d\lambda$ (for $f\in S(\mathcal{F})$), on the other, every $x^*\in S(\mathcal{F})^*$ defines on $\mathcal{F}$ the measure $\lambda_{x^*}(A):=x^*(\chi_A)$ (for $A\in\mathcal{F}$). See Topological Riesz spaces and measure theory (Fremlin) for a more complete reference.

Having showed that the set of probabilities over $\mathcal{F}$ is contained in the topological dual of the normed space $S(\mathcal{F})$, it is clear why we can talk about weak$^*$-convergence.

PS: It is worth observing that by the Stone representation Theorem for Boolean rings any Boolean ring $\mathcal{R}$ is isomorphic to the ring of clopen sets in a locally compact Hausdorff space (see Measure theory Vol III (Fremlin), or this survey by Tao). Following this line, the approach showed by Tomasz can be proved to be much closer to the one exposed so far than someone would think. It would be interesting to go through this idea.

As far as I know, the right context here is that you have a locally compact Hausdorff space $X$, which give you the Banach space $C_0(X)$ of continuous functions vanishing at infinity (i.e. the set of all $x$ such that $\lvert f(x)\rvert\geq \varepsilon$ is compact, for all $\varepsilon>0$), equipped with the supremum norm.

Notice that each (signed, or even complex) regular Borel measure of bounded variation gives you a continuous linear functional on $C_0(X)$ (via integration). By Riesz-Markov theorem, the converse is also true: every continuous functional on $C_0(X)$ is given by integration with a (unique) complex, regular Borel measure of bounded variation.

Thus $C_0(X)^*$ can be identified with the space of all complex, regular Borel measures of bounded variation, and thus you can put the weak and weak-$^*$ topology on the latter. In particular, you can see the regular Borel probability measures as a subset of $C_0(X)^*$ and endow them with the appropriate subspace topology.

I haven't heard of a definition of weak-$^*$ topology on the space of measures that does not somehow reference a topology on the underlying measure space. You could find an isomorphism with a topological measure space and pull the measure through it, but I don't think the weak-$^*$ topology would be independent of the choice of isomorphism (though I think weak topology might be).