Build bootstrap only in CMake for bpftool

Follow the Makefile to build bootstrap only for bpftool.

Signed-off-by: Xin Zhang <zhangxin.xa@gmail.com>

Author: xzhangxa

examples/c/CMakeLists.txt

@@ -29,7 +29,7 @@ ExternalProject_Add(bpftool
   PREFIX bpftool
   SOURCE_DIR ${CMAKE_CURRENT_SOURCE_DIR}/../../bpftool/src
   CONFIGURE_COMMAND ""
-  BUILD_COMMAND make
+  BUILD_COMMAND make bootstrap
     OUTPUT=${CMAKE_CURRENT_BINARY_DIR}/bpftool/
   BUILD_IN_SOURCE TRUE
   INSTALL_COMMAND ""
@@ -51,7 +51,7 @@ endif()
 
 # Set BpfObject input parameters -- note this is usually not necessary unless
 # you're in a highly vendored environment (like libbpf-bootstrap)
-set(BPFOBJECT_BPFTOOL_EXE ${CMAKE_CURRENT_BINARY_DIR}/bpftool/bpftool)
+set(BPFOBJECT_BPFTOOL_EXE ${CMAKE_CURRENT_BINARY_DIR}/bpftool/bootstrap/bpftool)
 set(BPFOBJECT_VMLINUX_H ${CMAKE_CURRENT_SOURCE_DIR}/../../vmlinux/vmlinux.h)
 set(LIBBPF_INCLUDE_DIRS ${CMAKE_CURRENT_BINARY_DIR}/libbpf)
 set(LIBBPF_LIBRARIES ${CMAKE_CURRENT_BINARY_DIR}/libbpf/libbpf.a)