bpftool: Add Makefile target bootstrap

This target is used to only build the bootstrap bpftool, which will be
used to generate bpf skeletons for other tools, like perf.

Signed-off-by: Song Liu <[email protected]>
Tested-by: Arnaldo Carvalho de Melo <[email protected]>
Acked-by: Namhyung Kim <[email protected]>
Cc: Alexander Shishkin <[email protected]>
Cc: Jiri Olsa <[email protected]>
Cc: Mark Rutland <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: [email protected]
Link: http://lore.kernel.org/lkml/[email protected]
Signed-off-by: Arnaldo Carvalho de Melo <[email protected]>

tools/bpf/bpftool/Makefile

@@ -148,6 +148,8 @@ VMLINUX_BTF_PATHS ?= $(if $(O),$(O)/vmlinux)				\
 		     /boot/vmlinux-$(shell uname -r)
 VMLINUX_BTF ?= $(abspath $(firstword $(wildcard $(VMLINUX_BTF_PATHS))))
 
+bootstrap: $(BPFTOOL_BOOTSTRAP)
+
 ifneq ($(VMLINUX_BTF)$(VMLINUX_H),)
 ifeq ($(feature-clang-bpf-co-re),1)