[multiboot] Allow for unspecified {load,bss}_end_addr for raw images

The multiboot specification states that, for raw images, if
load_end_addr is zero then it should be interpreted as meaning "use
the entire file", and if bss_end_addr is zero it should be interpreted
as meaning "no bss".

src/arch/i386/image/multiboot.c

 
 	/* Verify and prepare segment */
 	offset = ( hdr->offset - hdr->mb.header_addr + hdr->mb.load_addr );
-	filesz = ( hdr->mb.load_end_addr - hdr->mb.load_addr );
-	memsz = ( hdr->mb.bss_end_addr - hdr->mb.load_addr );
+	filesz = ( hdr->mb.load_end_addr ?
+		   ( hdr->mb.load_end_addr - hdr->mb.load_addr ) :
+		   ( image->len - offset ) );
+	memsz = ( hdr->mb.bss_end_addr ?
+		  ( hdr->mb.bss_end_addr - hdr->mb.load_addr ) : filesz );
 	buffer = phys_to_user ( hdr->mb.load_addr );
 	if ( ( rc = prep_segment ( buffer, filesz, memsz ) ) != 0 ) {
 		DBGC ( image, "MULTIBOOT %p could not prepare segment: %s\n",