* include/grub/powerpc/ieee1275/kernel.h: Introduce GRUB_MOD_GAP

to create a gap between _end and the modules added to the image
with grub-mkrescue.  That fixes "CLAIM failed" on PowerMAC.
* kern/powerpc/ieee1275/init.c: Use GRUB_MOD_GAP.
* util/elf/grub-mkimage.c (add_segments): Likewise.

kern/powerpc/ieee1275/init.c

@@ -242,5 +242,5 @@ grub_get_rtc (void)
 grub_addr_t
 grub_arch_modules_addr (void)
 {
-  return ALIGN_UP(_end, GRUB_MOD_ALIGN);
+  return ALIGN_UP(_end + GRUB_MOD_GAP, GRUB_MOD_ALIGN);
 }