lib: utils/fdt: Use kconfig for enabling/disabling

We update FDT support makefile to use kconfig for enabling/disabling.
To avoid compilation errors, we also enable FDT 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>