Makefile: Fix typo related to object.mk

The "object.mk" name referred in top-level makefile should be
"objects.mk".

Signed-off-by: Anup Patel <apatel@ventanamicro.com>
Reviewed-by: Andrew Jones <ajones@ventanamicro.com>
Tested-by: Andrew Jones <ajones@ventanamicro.com>
Acked-by: Atish Patra <atishp@rivosinc.com>
Tested-by: Atish Patra <atishp@rivosinc.com>