Merge branch 'samples-bpf-user-experience'

Jesper Dangaard Brouer says:

====================
samples/bpf: Improve user experience

It is a steep learning curve getting started with using the eBPF
examples in samples/bpf/.  There are several dependencies, and
specific versions of these dependencies.  Invoking make in the correct
manor is also slightly obscure.

This patchset cleanup, document and hopefully improves the first time
user experience with the eBPF samples directory by auto-detecting
certain scenarios.

V4:
 - Address Naveen's nitpicks
 - Handle/fail if extra args are passed in LLC or CLANG (David Laight)

V3:
 - Add Alexei's ACKs
 - Remove README paragraph about LLVM experimental BPF target
   as it only existed between LLVM version 3.6 to 3.7.

V2:
 - Adjusted recommend minimum versions to 3.7.1
 - Included clang build instructions
 - New patch adding CLANG variable and validation of command
====================

Signed-off-by: David S. Miller <davem@davemloft.net>