Fortunately you can easily hook one up using the EJTAG header. Connect a momentary switch between RST_N (pin 11) and GND (pin 2, 4, 6, 8 or 10). If your board is angled the same way as mine is in the picture, then the pin-out of the header matches that on the hardware page (linked above) — i.e. RST_N is second from bottom on the left and GND is third from bottom on the right.

I’ve got somewhat-reassuring confirmation from the CI20 mailing list that RST_N has a debounce circuit and a pull-up resistor, so a simple switch should be okay.