The previous build process ran sed against the linker scripts twice:
once to replace <X> with the contents of sym_x.ld, a second time to
correct any paths referencing tools/.
The first can be replaced with INCLUDE commands, as was done in
pret/pokeemerald@7c57699 (in 2018).
The second has been unnecessary since 942c248 (also 2018).
With both of these removed there is no longer a need to have a build
step for them, and $(ELF) can take the sym_x.ld files as prerequisites
instead.
