Constify received GDB packet

v2:
- Split work into separate patches

The received packet will not be altered in any of the processing functions.
Some it can be made "const".

Change-Id: I7bb410224cf6daa74a6c494624176ccb9ae638ac
Signed-off-by: Christian Eggers <ceggers@gmx.de>
Reviewed-on: http://openocd.zylin.com/1919
Tested-by: jenkins
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>

src/target/smp.h

@@ -21,7 +21,7 @@
 #include "server/server.h"
 
 int gdb_read_smp_packet(struct connection *connection,
-		char *packet, int packet_size);
+		char const *packet, int packet_size);
 int gdb_write_smp_packet(struct connection *connection,
-		char *packet, int packet_size);
+		char const *packet, int packet_size);