libbpf: Improve usability of libbpf Makefile

Current libbpf Makefile does not contain the help command, which
is inconvenient to use. Similar to the Makefile help command of the
perf, a help command is provided to list the commands supported by
libbpf make and the functions of the commands.
Signed-off-by: Xin Liu <liuxin350@huawei.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20221202081738.128513-1-liuxin350@huawei.com

tools/lib/bpf/Makefile

@@ -286,3 +286,20 @@ tags:
 
 # Delete partially updated (corrupted) files on error
 .DELETE_ON_ERROR:
+
+help:
+	@echo 'libbpf common targets:'
+	@echo '  HINT: use "V=1" to enable verbose build'
+	@echo '  all     - build libraries and pkgconfig'
+	@echo '  clean   - remove all generated files'
+	@echo '  check   - check abi and version info'
+	@echo ''
+	@echo 'libbpf install targets:'
+	@echo '  HINT: use "prefix"(defaults to "/usr/local") or "DESTDIR" (defaults to "/")'
+	@echo '        to adjust target desitantion, e.g. "make prefix=/usr/local install"'
+	@echo '  install          - build and install all headers, libraries and pkgconfig'
+	@echo '  install_headers  - install only headers to include/bpf'
+	@echo ''
+	@echo 'libbpf make targets:'
+	@echo '  tags    - use ctags to make tag information for source code browsing'
+	@echo '  cscope  - use cscope to make interactive source code browsing database'