gdb_server: support File-I/O Remote Protocol Extension

gdb_server: support File-I/O Remote Protocol Extension

The File I/O remote protocol extension allows the target to use the
host's file system and console I/O to perform various system calls.

To use the function, targets need to prepare two callback functions:
* get_gdb_finish_info: to get file I/O parameters from target
* gdb_fileio_end: pass file I/O response to target

As target is halted, gdb_server will try to get file-I/O information
from target through target_get_gdb_fileio_info(). If the callback function
returns ERROR_OK, gdb_server will initiate a file-I/O request to gdb.
After gdb finishes system call, gdb will pass response of the system call
to target through target_gdb_fileio_end() and continue to run(continue or step).

To implement the function, I add a new data structure in struct target,
called struct gdb_fileio_info, to record file I/O name and parameters.

Details refer to GDB manual "File-I/O Remote Protocol Extension"

Change-Id: I7f4d45e7c9e967b6d898dc79ba01d86bc46315d3
Signed-off-by: Hsiangkai Wang <hsiangkai@gmail.com>
Reviewed-on: http://openocd.zylin.com/1102
Tested-by: jenkins
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>