ext: Import libmetal for IPC/open-amp

Origin:
   https://github.com/OpenAMP/libmetal

Status:
   606c31438025b9fb1515dace1c642d5835d8d33c [v2018.04]

   When we import libmetal we removed the tests/ and examples/ dir to
   reduce the amount of code imported.

Purpose:
   HAL abstraction layer used by open-amp

Description:
   Libmetal provides common user APIs to access devices, handle device
   interrupts and request memory across the following operating
   environments:

   * Linux user space (based on UIO and VFIO support in the kernel)
   * RTOS (with and without virtual memory)
   * Bare-metal environments

Dependencies:
   Depends on Zephyr itself as it utilizes Zephyr's APIs to provide an
   abstraction to open-amp.

URL:
   https://github.com/OpenAMP/libmetal

commit:
   606c31438025b9fb1515dace1c642d5835d8d33c

Maintained-by:
   External

License:
   BSD-3-Clause

License Link:
   https://github.com/OpenAMP/libmetal/blob/master/LICENSE.md



Signed-off-by: Kumar Gala <kumar.gala@linaro.org>