bpf: Drop libbpf, libelf, libz dependency from bpf preload.

Drop libbpf, libelf, libz dependency from bpf preload.
This reduces bpf_preload_umd binary size
from 1.7M to 30k unstripped with debug info
and from 300k to 19k stripped.

Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Martin KaFai Lau <kafai@fb.com>
Link: https://lore.kernel.org/bpf/20220131220528.98088-8-alexei.starovoitov@gmail.com

kernel/bpf/preload/Makefile

@@ -1,40 +1,16 @@
 # SPDX-License-Identifier: GPL-2.0
 
 LIBBPF_SRCS = $(srctree)/tools/lib/bpf/
-LIBBPF_OUT = $(abspath $(obj))/libbpf
+LIBBPF_INCLUDE = $(LIBBPF_SRCS)/..
-LIBBPF_A = $(LIBBPF_OUT)/libbpf.a
-LIBBPF_DESTDIR = $(LIBBPF_OUT)
-LIBBPF_INCLUDE = $(LIBBPF_DESTDIR)/include
-
-# Although not in use by libbpf's Makefile, set $(O) so that the "dummy" test
-# in tools/scripts/Makefile.include always succeeds when building the kernel
-# with $(O) pointing to a relative path, as in "make O=build bindeb-pkg".
-$(LIBBPF_A): | $(LIBBPF_OUT)
-	$(Q)$(MAKE) -C $(LIBBPF_SRCS) O=$(LIBBPF_OUT)/ OUTPUT=$(LIBBPF_OUT)/   \
-		DESTDIR=$(LIBBPF_DESTDIR) prefix=			       \
-		$(LIBBPF_OUT)/libbpf.a install_headers
-
-libbpf_hdrs: $(LIBBPF_A)
-
-.PHONY: libbpf_hdrs
-
-$(LIBBPF_OUT):
-	$(call msg,MKDIR,$@)
-	$(Q)mkdir -p $@
 
 userccflags += -I $(srctree)/tools/include/ -I $(srctree)/tools/include/uapi \
 	-I $(LIBBPF_INCLUDE) -Wno-unused-result
 
 userprogs := bpf_preload_umd
 
-clean-files := libbpf/
-
-$(obj)/iterators/iterators.o: | libbpf_hdrs
-
 bpf_preload_umd-objs := iterators/iterators.o
-bpf_preload_umd-userldlibs := $(LIBBPF_A) -lelf -lz
 
-$(obj)/bpf_preload_umd: $(LIBBPF_A)
+$(obj)/bpf_preload_umd:
 
 $(obj)/bpf_preload_umd_blob.o: $(obj)/bpf_preload_umd