build: add support for using prebuilt LLVM toolchain

When the prebuilt llvm toolchain is unpacked into the source dir,
it is automatically picked up and used by the build system, and eBPF
based packages can be selected

Signed-off-by: Felix Fietkau <nbd@nbd.name>

include/bpf.mk

@@ -1,6 +1,7 @@
 BPF_DEPENDS := @HAS_BPF_TOOLCHAIN
+LLVM_VER:=
 
-ifneq ($(CONFIG_BPF_TOOLCHAIN_HOST),)
+ifneq ($(CONFIG_USE_LLVM_HOST),)
   BPF_TOOLCHAIN_HOST_PATH:=$(call qstrip,$(CONFIG_BPF_TOOLCHAIN_HOST_PATH))
   ifneq ($(BPF_TOOLCHAIN_HOST_PATH),)
     BPF_PATH:=$(BPF_TOOLCHAIN_HOST_PATH)/bin:$(PATH)
@@ -9,9 +10,12 @@ ifneq ($(CONFIG_BPF_TOOLCHAIN_HOST),)
   endif
   CLANG:=$(firstword $(shell PATH='$(BPF_PATH)' which clang clang-13 clang-12 clang-11))
   LLVM_VER:=$(subst clang,,$(notdir $(CLANG)))
-else
+endif
+ifneq ($(CONFIG_USE_LLVM_PREBUILT),)
+  CLANG:=$(TOPDIR)/llvm-bpf/bin/clang
+endif
+ifneq ($(CONFIG_USE_LLVM_BUILD),)
   CLANG:=$(STAGING_DIR_HOST)/llvm-bpf/bin/clang
-  LLVM_VER:=
 endif
 
 LLVM_PATH:=$(dir $(CLANG))