datapath: Use ccflags-y instead of deprecated EXTRA_CFLAGS

This allows users to pass in additional compiler flags through the
environment variable EXTRA_CFLAGS, e.g.

   make EXTRA_CFLAGS=-Wno-error=foo V=1

Reported-by: Alexandru Ardelean <ardeleanalex@gmail.com>
Signed-off-by: Thomas Graf <tgraf@noironetworks.com>
travis: Extended kernel build matrix

When doing test builds, build against the following kernels:
  - KERNEL=3.17.4
  - KERNEL=3.16.7
  - KERNEL=3.14.25
  - KERNEL=3.12.33
  - KERNEL=3.10.61
  - KERNEL=3.4.104
  - KERNEL=2.6.32.64

Signed-off-by: Thomas Graf <tgraf@noironetworks.com>