esp8266: Implement software SPI class.

Supports speeds up to 500k baud, polarity=0/1, phase=0/1, and using any
pins.  Only supports MSB output at the moment.
diff --git a/esp8266/esp8266.ld b/esp8266/esp8266.ld
index 2677668..8430195 100644
--- a/esp8266/esp8266.ld
+++ b/esp8266/esp8266.ld
@@ -133,6 +133,7 @@
         *modpybpin.o(.literal*, .text*)
         *modpybrtc.o(.literal*, .text*)
         *modpybadc.o(.literal*, .text*)
+        *modpybspi.o(.literal*, .text*)
         *modesp.o(.literal* .text*)
         *modnetwork.o(.literal* .text*)
         *moduos.o(.literal* .text*)