riscv: Fix build dependency for loader

The Makefile addition for the flat image loader missed an obj prefix.

For most parallel builds this worked out fine, but with -j1 the dependency
wasn't fulfilled and thus fails:

arch/riscv/boot/loader.S: Assembler messages:
arch/riscv/boot/loader.S:7: Error: file not found: arch/riscv/boot/Image

Fixes: 405fe7aa ("riscv: provide a flat image loader")
Cc: Christoph Hellwig <hch@lst.de>
Signed-off-by: Olof Johansson <olof@lixom.net>
Reviewed-by: Anup Patel <anup@brainfault.org>
Signed-off-by: Paul Walmsley <paul.walmsley@sifive.com>