lib: utils/serial: Use kconfig for enabling/disabling drivers

We update serial drivers makefile to use kconfig for enabling/disabling
drivers. To avoid compile errors, we also enable appropriate serial
drivers for each platform.

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