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>

include/bpf.mk

@@ -1,4 +1,4 @@
-BPF_DEPENDS := @HAS_BPF_TOOLCHAIN
+BPF_DEPENDS := @HAS_BPF_TOOLCHAIN +@NEED_BPF_TOOLCHAIN
 LLVM_VER:=
 
 CLANG_MIN_VER:=12