thermistor: Add support for defining custom thermistors

Add the ability to define a new thermistor type in the config file.

Signed-off-by: Kevin O'Connor <kevin@koconnor.net>

config/example-extras.cfg

@@ -394,6 +394,31 @@
 #   provided when using an st7920 display.
 
 
+# Custom thermistors (one may define any number of sections with a
+# "thermistor" prefix).  A custom thermistor may be used in the
+# sensor_type field of a heater config section. (For example, if one
+# defines a "[thermistor my_thermistor]" section then one may use a
+# "sensor_type: my_thermistor" when defining a heater.) Be sure to
+# place the thermistor section in the config file above its first use
+# in a heater section.
+#[thermistor my_thermistor]
+#temperature1:
+#resistance1:
+#temperature2:
+#resistance2:
+#temperature3:
+#resistance3:
+#   Three resistance measurements (in Ohms) at the given temperatures
+#   (in Celsius). The three measurements will be used to calculate the
+#   Steinhart-Hart coefficients for the thermistor.  These parameters
+#   must be provided when using Steinhart-Hart to define the
+#   thermistor.
+#beta:
+#   Alternatively, one may define temperature1, resistance1, and beta
+#   to define the thermistor parameters. This parameter must be
+#   provided when using "beta" to define the thermistor.
+
+
 # Replicape support - see the generic-replicape.cfg file for further
 # details.
 #[replicape]