begin splitting up source files, so we can have a tree of sources...

unique to each architecture.  For now, we have "esp32" and "bare"

esp32 is the old esp stuff

bare is an target suitable for emulation that doesn't require any
particular hardware to run (no bluetooth, no i2c devices, no spi devices)

src/target_specific.h

@@ -0,0 +1,6 @@
+#pragma once
+
+// Functions that are unique to particular target types (esp32, bare, nrf52 etc...)
+
+// Enable/disable bluetooth.
+void setBluetoothEnable(bool on);