GKI cleanup - moved GKI buffer allocation wrappers to OSI

GKI cleanup - moved GKI buffer allocation wrappers to OSI

* Moved the following GKI buffer allocation functions to OSI:
  - GKI_getbuf() -> osi_getbuf()
  - GKI_freebuf() -> osi_freebuf()
  - GKI_get_buf_size() -> osi_get_buf_size()

  For now we need the osi_getbuf() / osi_freebuf() allocation wrapper,
  because we need to be able to call osi_get_buf_size() on the allocated
  buffer.
  In the future those should be replaced with osi_malloc() / osi_free().
  Currently, the osi_malloc() buffer size internal allocation tracker
 does not always track the size, hence we need the osi_getbuf() wrapper.

* Replaced GKI_MAX_BUF_SIZE with BT_DEFAULT_BUFFER_SIZE

* Added new file include/bt_common.h that can be usee to include
  few files that should be included alost everywhere (e.g. bt_target.h"
  NOTE: This file might be removed in the future and we should include
  everywhere the right set of header files.

* Removed some of the GKI-related references

* Removed file include/gki_target.h

Change-Id: Ie87830e73143de200746d54235aa99f228a95024