scripts: Cover sifive/fu540 in the 32-bit build

We now support building sifive/fu540 as a 32-bit platform. Add it
in the 32-bit platform list of binary archive script.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Anup Patel <anup.patel@wdc.com>