Revert abs -> ABS change from commit 0dab6926b3.

The avr-ravenlcd cannot include sys/cc.h since this in turns includes contiki-conf.h
which the avr-ravenlcd does not have.

platform/avr-ravenlcd/lcd.c

@@ -411,7 +411,7 @@ lcd_num_putdec(int numb, lcd_padding_t padding)
     }
 
     /* Convert to BCD */
-    bcd = itobcd(ABS(numb));
+    bcd = itobcd(abs(numb));
 
     /* Print */
     return lcd_num_print(bcd, (bool)(numb<0), padding);