Fix incorrect signature (from avr-libc) for ATmega644p.

Fix incorrect BoD fuses in Makefile for several targets.
http://code.google.com/p/optiboot/issues/detail?id=61

optiboot/bootloaders/optiboot/Makefile

@@ -233,7 +233,7 @@ atmega328_isp: HFUSE = DE
 # Low power xtal (16MHz) 16KCK/14CK+65ms
 atmega328_isp: LFUSE = FF
 # 2.7V brownout
-atmega328_isp: EFUSE = 05
+atmega328_isp: EFUSE = FD
 atmega328_isp: isp
 
 atmega1284: TARGET = atmega1284p
@@ -273,7 +273,7 @@ sanguino_isp: HFUSE = DE
 # Low power xtal (16MHz) 16KCK/14CK+65ms
 sanguino_isp: LFUSE = FF
 # 2.7V brownout
-sanguino_isp: EFUSE = 05
+sanguino_isp: EFUSE = FD
 sanguino_isp: isp
 
 # Mega has a minimum boot size of 1024 bytes, so enable extra functions
@@ -293,7 +293,7 @@ mega_isp: HFUSE = DE
 # Low power xtal (16MHz) 16KCK/14CK+65ms
 mega_isp: LFUSE = FF
 # 2.7V brownout
-mega_isp: EFUSE = 05
+mega_isp: EFUSE = FD
 mega_isp: isp
 
 # ATmega8

optiboot/bootloaders/optiboot/optiboot.c

@@ -284,6 +284,9 @@ void appStart() __attribute__ ((naked));
 #elif defined (__AVR_ATmega644P__)
 #define RAMSTART (0x100)
 #define NRWWSTART (0xE000)
+// correct for a bug in avr-libc
+#undef SIGNATURE_2
+#define SIGNATURE_2 0x0A
 #elif defined (__AVR_ATmega1284P__)
 #define RAMSTART (0x100)
 #define NRWWSTART (0xE000)