Serial

After a bit of research using a logic analyzer, the following appears to be the pinout of the 6 pin UART header (JP1). Default settings are baud: 115200, 8 bit, no parity, 1 stop bit.

Pin 1  3.3 V DC
Pin 2  ??
Pin 3  S_in
Pin 4  ??
Pin 5  S_out
Pin 6  GND

JTAG

The JTAG header is a standard EJTAG header. From Wikipedia: EJTAG 2.5 and later defines a dual row 14-pin male header (mechanical connector) with 0.1"x0.1" spacing to attach a JTAG probe. 3M part number 2414-600UB is recommended.

nTRST  1  2 GND
TDI    3  4 GND
TDO    5  6 GND
TMS    7  8 GND
TCK    9 10 GND
nSRST 11 12 -key
DINT  13 14 VCC

Debug mode

To jump into the debugger/boot loader, hold the reset button at startup for a short time (less than half a second?). You will get a DISH> prompt.

HEAP: addr=800740C0 size=03F8BF40
Hello, this is CREAM for AR9130![LOADER]
'CAKE' CREAM Kernel debugger Ver 1.4.2
'HeDEB' Heap debugger Ver 1.0.0 
'DISH' Debugging shell Version 2.5.0