gdb_server: fix extended_protocol for multi-target

gdb_server: fix extended_protocol for multi-target

The flag extended_protocol is currently a single static variable
thus, in case of multiple targets, it is shared among all the gdb
connections. This is an issue if the gdb connections are not all
using extended protocol, but also when one connection get closed
because the code sets the flag to zero impacting the other
connections still open.

Move the flag extended_protocol in the per-connection struct
gdb_connection.

Change-Id: I19d565f925df6a31767fd8d392242f60867109f2
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: http://openocd.zylin.com/5310
Tested-by: jenkins
Reviewed-by: Moritz Fischer <moritzf@google.com>
Reviewed-by: Tomas Vanek <vanekt@fbl.cz>