Add battery sensing (mostly) for TBEAM0.7

However, disabled until someone with suitable hardware can test and report
back.

@slavino and @tschundler would you be willing to try it with your boards?

You'll need to uncomment the following line in configuration.h

// #define BATTERY_PIN 35 // A battery voltage measurement pin, voltage divider connected here to measure battery voltage

src/configuration.h

@@ -167,7 +167,6 @@ along with this program.  If not, see <http://www.gnu.org/licenses/>.
 
 // Leave undefined to disable our PMU IRQ handler
 #define PMU_IRQ 35
-
 #define AXP192_SLAVE_ADDRESS 0x34
 
 #elif defined(TBEAM_V07)
@@ -180,6 +179,7 @@ along with this program.  If not, see <http://www.gnu.org/licenses/>.
 #define I2C_SCL 22
 
 #define BUTTON_PIN 39
+// #define BATTERY_PIN 35 // A battery voltage measurement pin, voltage divider connected here to measure battery voltage
 
 #ifndef USE_JTAG
 #define RF95_RESET 23
@@ -278,6 +278,8 @@ along with this program.  If not, see <http://www.gnu.org/licenses/>.
 #define GPS_RX_PIN 36
 #define GPS_TX_PIN 39
 
+// #define BATTERY_PIN 35 // A battery voltage measurement pin, voltage divider connected here to measure battery voltage
+
 #define I2C_SDA 21 // I2C pins for this board
 #define I2C_SCL 22