riscv: boot: Fix creation of loader.bin

When flashing loader.bin for K210 using kflash:

    [ERROR] This is an ELF file and cannot be programmed to flash directly: arch/riscv/boot/loader.bin

Before, loader.bin relied on "OBJCOPYFLAGS := -O binary" in the main
RISC-V Makefile to create a boot image with the right format.  With this
removed, the image is now created in the wrong (ELF) format.

Fix this by adding an explicit rule.

Fixes: 505b0295

 ("riscv: Remove duplicate objcopy flag")
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Damien Le Moal <dlemoal@kernel.org>
Link: https://lore.kernel.org/r/1086025809583809538dfecaa899892218f44e7e.1698159066.git.geert+renesas@glider.be
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>