linux/linux-machine-kconfig.inc: Refactor out kconfig merging

* Move the kconfig merging task into separate include
* Update the merging to use "scripts/kconfig/merge_config.sh" instead of
  concatenating files together

Signed-off-by: Nathan Rossi <nathan.rossi@xilinx.com>
linux/linux-machine-common.inc: Refactor defconfigs

* Refactor defconfig default setting of MACHINE_KCONFIG, moving it from
  linux-xlnx.inc to linux-machine-common.inc, to allow sharing between
  linux-xlnx and linux-yocto recipes
* Switch to prepending the configs, this ensures that the defconfigs are
  at the lowest priority, and that additional configs appended will
  override the defconfig

Signed-off-by: Nathan Rossi <nathan.rossi@xilinx.com>
machine-xilinx-default.inc: Move defconfig setup

* Move the defconfig into machine-xilinx-default.inc, so that the
  defconfigs can be completely overridden from machine configs
* Add 3.14 defconfigs

Signed-off-by: Nathan Rossi <nathan.rossi@xilinx.com>
machine/boards: Fix up cfg fragment eof newlines

* Make sure that cfg fragments have newlines at the end of the file

Signed-off-by: Nathan Rossi <nathan.rossi@xilinx.com>