Issue 54: Code grows to >512 bytes with gcc 4.5.3

Use attribute "OS_main" instead of "naked" for main()
http://code.google.com/p/optiboot/issues/detail?id=54

This is more correct, and prevents code growth when using
gcc4.5.3 and later, where certain optimizations are disabled
in "naked" functions.  In 4.3.2 (current used, the binary
does not change.
See http://gcc.gnu.org/bugzilla/show_bug.cgi?id=42240

optiboot/bootloaders/optiboot/optiboot.c

@@ -133,6 +133,12 @@
 /* Edit History:					  */
 /*							  */
 /* Jan 2012:                                              */
+/* 4.4 WestfW: use attribute OS_main instead of naked for */
+/*             main().  This allows optimizations that we */
+/*             count on, which are prohibited in naked    */
+/*             functions due to PR42240.  (keeps us less  */
+/*             than 512 bytes when compiler is gcc4.5     */
+/*             (code from 4.3.2 remains the same.)        */
 /* 4.4 WestfW and Maniacbug:  Add m1284 support.  This    */
 /*             does not change the 328 binary, so the     */
 /*             version number didn't change either. (?)   */
@@ -239,7 +245,7 @@ asm("  .section .version\n"
 /* The main function is in init9, which removes the interrupt vector table */
 /* we don't need. It is also 'naked', which means the compiler does not    */
 /* generate any entry or exit code itself. */
-int main(void) __attribute__ ((naked)) __attribute__ ((section (".init9")));
+int main(void) __attribute__ ((OS_main)) __attribute__ ((section (".init9")));
 void putch(char);
 uint8_t getch(void);
 static inline void getNch(uint8_t); /* "static inline" is a compiler hint to reduce code size */