atusb/fw: introduced function flash_end_write to properly finish flashing

- dfu.h (flash_end_write), flash.c (flash_end_write): write any incomplete
  buffer
- flash.c (flash_write, flash_end_write): call boot_rww_enable only at the
  very end, it won't erase our buffer in mid-page
- usb/dfu.c (my_setup): call flash_end_write at the end of a download

atusb/fw/flash.c

@@ -57,7 +57,15 @@ void flash_write(const uint8_t *buf, uint16_t size)
 			boot_spm_busy_wait();
 		}
         }
+}
 
+
+void flash_end_write(void)
+{
+	if (payload & (SPM_PAGESIZE-1)) {
+		boot_page_write(payload & ~(SPM_PAGESIZE-1));
+		boot_spm_busy_wait();
+	}
 	boot_rww_enable();
 }