firmware: payload: test: Change to SBI v2.0 DBCN ecalls

As the the "Console Putchar" extension is already legacy and may
be removed in the furture. So replace it with the SBI v2.0 "DBCN"
extension.

Signed-off-by: Inochi Amaoto <inochiama@outlook.com>
Reviewed-by: Anup Patel <anup@brainfault.org>