MIPS: zboot: put appended dtb into a section

This will make a separated section for dtb appear in ELF, and we can
then use objcopy to patch a dtb into vmlinuz when RAW_APPENDED_DTB
is set in kernel config.

command to patch a dtb:
objcopy --set-section-flags=.appended_dtb=alloc,contents \
        --update-section=.appended_dtb=<target>.dtb vmlinuz

Signed-off-by: Chuanhong Guo <gch981213@gmail.com>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>

arch/mips/boot/compressed/ld.script

@@ -31,9 +31,12 @@ SECTIONS
 		CONSTRUCTORS
 		. = ALIGN(16);
 	}
-	__appended_dtb = .;
-	/* leave space for appended DTB */
-	. += 0x100000;
+
+	.appended_dtb : {
+		__appended_dtb = .;
+		/* leave space for appended DTB */
+		. += 0x100000;
+	}
 
 	_edata = .;
 	/* End of data section */