Geoff's answer and the comments might be clarified a little as follows. You are looking at rational points of a parabolic subgroup over a finite field (where it's not important whether the algebraic group is of classical type or not). In the split (Chevalley group) case, it's easy to check orders of the various finite groups involved. Since a proper parabolic group is just the product of a central algebraic torus and a lower rank semisimple group, the rational points sometimes yield a solvable group even if the original parabolic is not itself a solvable algebraic group (a Borel subgroup). To assemble a complete list of possibilities, look at the simple algebraic groups having solvable groups of rational points over a field of small characteristic. The torus part is already commutative, so the product can be solvable.
Naturally it gets more complicated to treat all types of twisted groups as well as Suzuki or Ree groups, but in principle it looks reasonable to assemble a complete list of solvable finite parabolics.
[Terminology: for me language like "the Borel subgroup" is a bit jarring, since there are many conjugate choices starting with the algebraic group.]