riscv: Remove duplicate objcopy flag

There are two duplicate `-O binary` flags when objcopying from vmlinux
to Image/xipImage.

RISC-V set `-O binary` flag in both OBJCOPYFLAGS in the top-level riscv
Makefile and OBJCOPYFLAGS_* in the boot/Makefile, and the objcopy cmd
in Kbuild would join them together.

The `-O binary` flag is only needed for objcopying Image, so remove the
OBJCOPYFLAGS in the top-level riscv Makefile.

Fixes: c0fbcd99

 ("RISC-V: Build flat and compressed kernel images")
Signed-off-by: Song Shuai <songshuaishuai@tinylab.org>
Reviewed-by: Palmer Dabbelt <palmer@rivosinc.com>
Link: https://lore.kernel.org/r/20230914091334.1458542-1-songshuaishuai@tinylab.org
Cc: stable@vger.kernel.org
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>