Add explicit #define for bogus temp override

The code to ignore the "bad thermistor reading failsafe"
suicide function depends on the existing of the PS_ON pin
feature.  But in some boards this shouldn't be the case
Fix this by adding an explicit definition to make our
intentions more clear and separable.

Marlin/pins.h

 #endif
 #define PS_ON_PIN 15
 
+// Gen 1.3 and earlier supplied thermistor power via PS_ON
+// Need to ignore the bad thermistor readings on those units
+#define BOGUS_TEMPERATURE_FAILSAFE_OVERRIDE
 //our pin for debugging.
 #define DEBUG_PIN 0
 

Marlin/temperature.cpp

     for(unsigned char e = 0; e < EXTRUDERS; e++) {
        if(current_raw[e] >= maxttemp[e]) {
           target_raw[e] = 0;
-          #if (PS_ON != -1)
+          #ifndef BOGUS_TEMPERATURE_FAILSAFE_OVERRIDE
           {
             max_temp_error(e);
             kill();;
        }
        if(current_raw[e] <= minttemp[e]) {
           target_raw[e] = 0;
-          #if (PS_ON != -1)
+          #ifndef BOGUS_TEMPERATURE_FAILSAFE_OVERRIDE
           {
             min_temp_error(e);
             kill();