platform: generic: Use kconfig for enabling/disabling overrides

We update generic platform to use kconfig for enabling/disabling
platform overrides. We also enable all platform overrides in generic
platform defconfig.

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>