samples/bpf: adjust Makefile and README.rst

Side effect of some kbuild changes resulted in breaking the
documented way to build samples/bpf/.

This patch change the samples/bpf/Makefile to work again, when
invoking make from the subdir samples/bpf/. Also update the
documentation in README.rst, to reflect the new way to build.

Signed-off-by: Jesper Dangaard Brouer <[e-mail]>
Acked-by: Daniel Borkmann <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

samples/bpf/Makefile

···
 TPROGLDLIBS_xdpsock		+= -pthread
 
 # Allows pointing LLC/CLANG to a LLVM backend with bpf support, redefine on cmdline:
-#  make samples/bpf/ LLC=~/git/llvm/build/bin/llc CLANG=~/git/llvm/build/bin/clang
+#  make M=samples/bpf/ LLC=~/git/llvm/build/bin/llc CLANG=~/git/llvm/build/bin/clang
 LLC ?= llc
 CLANG ?= clang
 LLVM_OBJCOPY ?= llvm-objcopy
···
 
 # Trick to allow make to be run from this directory
 all:
-	$(MAKE) -C ../../ $(CURDIR)/ BPF_SAMPLES_PATH=$(CURDIR)
+	$(MAKE) -C ../../ M=$(CURDIR) BPF_SAMPLES_PATH=$(CURDIR)
 
 clean:
 	$(MAKE) -C ../../ M=$(CURDIR) clean

samples/bpf/README.rst

···
 For building the BPF samples, issue the below command from the kernel
 top level directory::
 
- make samples/bpf/
-
-Do notice the "/" slash after the directory name.
+ make M=samples/bpf
 
 It is also possible to call make from this directory.  This will just
-hide the the invocation of make as above with the appended "/".
+hide the invocation of make as above.
 
 Manually compiling LLVM with 'bpf' support
 ------------------------------------------
···
 It is also possible to point make to the newly compiled 'llc' or
 'clang' command via redefining LLC or CLANG on the make command line::
 
- make samples/bpf/ LLC=~/git/llvm/build/bin/llc CLANG=~/git/llvm/build/bin/clang
+ make M=samples/bpf LLC=~/git/llvm/build/bin/llc CLANG=~/git/llvm/build/bin/clang
 
 Cross compiling samples
 -----------------------
···
 in its targets appropriate arm64 arch (usually it has several arches).
 Build samples::
 
- make samples/bpf/
+ make M=samples/bpf
 
 Or build samples with SYSROOT if some header or library is absent in toolchain,
 say libelf, providing address to file system containing headers and libs,
 can be RFS of target board::
 
- make samples/bpf/ SYSROOT=~/some_sysroot
+ make M=samples/bpf SYSROOT=~/some_sysroot