samples/bpf: allow make to be run from samples/bpf/ directory

It is not intuitive that 'make' must be run from the top level
directory with argument "samples/bpf/" to compile these eBPF samples.

Introduce a kbuild make file trick that allow make to be run from the
"samples/bpf/" directory itself.  It basically change to the top level
directory and call "make samples/bpf/" with the "/" slash after the
directory name.

Also add a clean target that only cleans this directory, by taking
advantage of the kbuild external module setting M=$PWD.
Signed-off-by: Jesper Dangaard Brouer <brouer@redhat.com>
Acked-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

samples/bpf/Makefile

@@ -85,6 +85,14 @@ HOSTLOADLIBES_test_overhead += -lelf -lrt
 #  make samples/bpf/ LLC=~/git/llvm/build/bin/llc
 LLC ?= llc
 
+# Trick to allow make to be run from this directory
+all:
+	$(MAKE) -C ../../ $$PWD/
+
+clean:
+	$(MAKE) -C ../../ M=$$PWD clean
+	@rm -f *~
+
 # Verify LLVM compiler is available and bpf target is supported
 .PHONY: verify_cmd_llc verify_target_bpf
 

samples/bpf/README.rst

@@ -36,6 +36,9 @@ top level directory::
 
 Do notice the "/" slash after the directory name.
 
+It is also possible to call make from this directory.  This will just
+hide the the invocation of make as above with the appended "/".
+
 Manually compiling LLVM with 'bpf' support
 ------------------------------------------