Introduce PLATFORM_ENTER, replacing the old PLATFORM_SETUP for DFU mode

setup. PLATFORM_SETUP now performs early general platform setup, such as
turning off the watchdog.

- fw/boot/boot.c (main): the first thing we do is to run PLATFORM_SETUP
- fw/boot/boot.c (boot_loader): instead of PLATFORM_SETUP, run
  PLATFORM_ENTER

fw/boot/boot.c

@@ -56,6 +56,10 @@
 #define	PLATFORM_SETUP
 #endif
 
+#ifndef PLATFORM_ENTER
+#define	PLATFORM_ENTER
+#endif
+
 #ifndef PLATFORM_EXIT
 #define	PLATFORM_EXIT
 #endif
@@ -174,7 +178,7 @@ static void boot_loader(void)
 
 	printk("%s #%u\n", build_date, build_number);
 
-	PLATFORM_SETUP;
+	PLATFORM_ENTER;
 
 	dfu_init();
 	usb_init();
@@ -205,6 +209,11 @@ static void boot_loader(void)
 
 void main(void)
 {
+	/*
+	 * Any early platform setup, such as turning off the watch dog.
+	 */
+	PLATFORM_SETUP;
+
 	/*
 	 * VDD monitor enable sequence, section 7.2
 	 *