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]

config/example.cfg

@@ -146,8 +146,9 @@ heater_pin: ar10
 #   periods) to the heater. The default is 1.0.
 sensor_type: EPCOS 100K B57560G104F
 #   Type of sensor - this may be "EPCOS 100K B57560G104F", "ATC
-#   Semitec 104GT-2", "NTC 100K beta 3950", or "AD595". This parameter
-#   must be provided.
+#   Semitec 104GT-2", "NTC 100K beta 3950", or "AD595". Additional
+#   sensor types may be available - see the example-extras.cfg file
+#   for details. This parameter must be provided.
 sensor_pin: analog13
 #   Analog input pin connected to the sensor. This parameter must be
 #   provided.