make snap-pkg support

make snap-pkg support

BugLink: http://bugs.launchpad.net/bugs/1700747
This patch integrates snapcraft in the kbuild environment of the Linux kernel.

To use it:

make defconfig
make snap-pkg

or in case of cross-compilation (e.g. arm):

export ARCH=arm; export CROSS_COMPILE=arm-linux-gnueabihf-
make defconfig
make snap-pkg

The resulting kernel snap will be generated in $(objtree)/snap

Signed-off-by: Paolo Pisati <paolo.pisati@canonical.com>
Acked-by: Stefan Bader <stefan.bader@canonical.com>
Acked-by: Thadeu Lima de Souza Cascardo <cascardo@canonical.com>
Signed-off-by: Seth Forshee <seth.forshee@canonical.com>