VxWorks 7 uses the same boot interface as the Linux kernel on Arm
(64-bit only), PowerPC and RISC-V architectures. Add logic to set
is_linux to true for VxWorks uImage for these architectures in
load_uboot_image().

Signed-off-by: Bin Meng <bin.meng@windriver.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-Id: <20220324134812.541274-2-bmeng.cn@gmail.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>