build: conditionally build llvm bpf toolchain by default

build: conditionally build llvm bpf toolchain by default

Unless another toolchain is present (or selected), build the bpf toolchain
whenever a package is selected that needs it.

Signed-off-by: Felix Fietkau <nbd@nbd.name>
(cherry picked from commit 6605e45676815156f0c47d4117ee401e6616fcac)