bsps/powerpc: Set output format in linker script

Set the proper output format in the linker script for the bootloader.
Otherwise the generic linker is used since Binutils 2.28 which does not
understand the PowerPC-specific relocations.

c/src/lib/libbsp/powerpc/shared/bootloader/ppcboot.lds

@@ -1,5 +1,5 @@
 OUTPUT_ARCH(powerpc)
-/* OUTPUT_FORMAT(ppcboot) */
+OUTPUT_FORMAT ("elf32-powerpc", "elf32-powerpc", "elf32-powerpc")
 /* Do we need any of these for elf?
    __DYNAMIC = 0;    */
 SECTIONS