Wrong type in function puthex() of file ATmegaBOOT_168.c

The function void puthex(char ch) of file
arduino-1.6.9/hardware/arduino/avr/bootloaders/atmega/ATmegaBOOT_168.c is wrong. The correct version is:
void puthex(unsigned char ch) {
unsigned char ah;

```
ah = ch >> 4;
if(ah >= 0x0a) {
ah = ah - 0x0a + 'a';
} else {
ah += '0';
}

ch &= 0x0f;
if(ch >= 0x0a) {
ch = ch - 0x0a + 'a';
} else {
ch += '0';
}

putch(ah);
putch(ch);
```

}
The type of ch and ah has to be unsigned char instead of char.