Makefile: build and use lightweight bootstrap version of bpftool

We don't need most of bpftool functionality, just the skeleton
generation and, potentially, static linking, so there is no point in
building full-featured final bpftool version. So build and use bootstrap
bpftool. This should also ease user's pain like [0].

  [0] libbpf#89

Cc: Quentin Monnet <quentin@isovalent.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>

examples/c/Makefile

@@ -5,7 +5,8 @@ LLVM_STRIP ?= llvm-strip
 LIBBPF_SRC := $(abspath ../../libbpf/src)
 BPFTOOL_SRC := $(abspath ../../bpftool/src)
 LIBBPF_OBJ := $(abspath $(OUTPUT)/libbpf.a)
-BPFTOOL ?= $(abspath $(OUTPUT)/bpftool/bpftool)
+BPFTOOL_OUTPUT ?= $(abspath $(OUTPUT)/bpftool)
+BPFTOOL ?= $(BPFTOOL_OUTPUT)/bootstrap/bpftool
 LIBBLAZESYM_SRC := $(abspath ../../blazesym/)
 LIBBLAZESYM_OBJ := $(abspath $(OUTPUT)/libblazesym.a)
 LIBBLAZESYM_HEADER := $(abspath $(OUTPUT)/blazesym.h)
@@ -70,7 +71,7 @@ clean:
 	$(call msg,CLEAN)
 	$(Q)rm -rf $(OUTPUT) $(APPS)
 
-$(OUTPUT) $(OUTPUT)/libbpf $(dir $(BPFTOOL)):
+$(OUTPUT) $(OUTPUT)/libbpf $(BPFTOOL_OUTPUT):
 	$(call msg,MKDIR,$@)
 	$(Q)mkdir -p $@
 
@@ -83,9 +84,9 @@ $(LIBBPF_OBJ): $(wildcard $(LIBBPF_SRC)/*.[ch] $(LIBBPF_SRC)/Makefile) | $(OUTPU
 		    install
 
 # Build bpftool
-$(BPFTOOL): | $(dir $(BPFTOOL))
+$(BPFTOOL): | $(BPFTOOL_OUTPUT)
 	$(call msg,BPFTOOL,$@)
-	$(Q)$(MAKE) ARCH= CROSS_COMPILE= OUTPUT=$(dir $(BPFTOOL)) -C $(BPFTOOL_SRC)
+	$(Q)$(MAKE) ARCH= CROSS_COMPILE= OUTPUT=$(BPFTOOL_OUTPUT)/ -C $(BPFTOOL_SRC) bootstrap
 
 
 $(LIBBLAZESYM_SRC)/target/release/libblazesym.a::