tools/btatt: Add "write-command" command.

Added command for the "Write" command.
src/shared/att: Recreate buffer when MTU gets reset.

Fixed bt_att_set_mtu to reallocate the given bt_att's internal buffer.
src/shared/att: Add "Signed Write" command.

This patch adds support for the "Signed Write" command and its parameter
structure.
unit/test-att: Add unit test for the "Signed Write" command.

Added test cases for the "Signed Write" command.
tools/btatt: Add "signed-write-command" command.

Added command for the "Signed Write" command.
src/shared/att: Add "Prepare Write" request and response.

This patch adds support for the "Prepare Write" request and response
with the corresponding parameters structures.
unit/test-att: Add unit test for "Prepare Write" request/response.

Added test cases for the "Prepare Write" request/response.
tools/btatt: Add "prepare-write" command.

Added command for the "Prepare Write" request.
src/shared/att: Add "Execute Write" request and response.

This patch add support for the "Execute Write" request and response with
the corresponding parameters structures.
unit/test-att: Add unit test for "Execute Write" request/response.

Added a test case for the "Execute Write" request/response.
tools/btatt: Add "execute-write" command.

Added command for the "Execute Write" request.