PPC: fix COFF bootloader entry point

The concept of entry point in COFF is actually different than in ELF.

In COFF, the entry point is actually a "descriptor" (pointer) to the actual
start code. So we patch the entry point address when calling objcopy.

Now my old Performa 5400/180 actually starts the loader correctly \o/

src/system/boot/Jamfile

@@ -93,8 +93,14 @@ rule BuildCoffLoader {
 
 actions BuildCoffLoader bind HACK_COFF {
 	rm -f $(1)
-	$(TARGET_OBJCOPY_$(TARGET_PACKAGING_ARCH)) -O $(COFF_FORMAT) $(2) $(1)
+	# get the address of the COFF entry
+	$(TARGET_OBJCOPY_$(TARGET_PACKAGING_ARCH)) -O symbolsrec $(2) $(2).syms
+	EP=`grep _coff_start $(2).syms | tr -d '\r' | cut -d'$' -f2`
+	rm -f $(2).syms
+	# copy to XCOFF format and patch the entry point
+	$(TARGET_OBJCOPY_$(TARGET_PACKAGING_ARCH)) -O $(COFF_FORMAT) --set-start="0x${EP}" $(2) $(1)
 	#$(CP) $(2) $(1)
+	# fill-in some fields objcopy missed
 	$(HACK_COFF) $(1)
 }
 

src/system/boot/platform/openfirmware/start.cpp

@@ -27,6 +27,10 @@ extern "C" void _start(uint32 _unused1, uint32 _unused2,
 	void *openFirmwareEntry);
 extern "C" void start(void *openFirmwareEntry);
 
+// XCOFF "entry-point" is actually a pointer to the real code
+extern "C" void *_coff_start;
+void *_coff_start = (void *)&_start;
+
 // GCC defined globals
 extern void (*__ctor_list)(void);
 extern void (*__ctor_end)(void);