Makefile: Add initial kconfig support for each platform

We extend the top-level makefile to allow kconfig based configuration
for each platform where each platform has it's own set of configs with
"defconfig" being the default config.

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>

platform/andes/ae350/Kconfig

@@ -0,0 +1,5 @@
+# SPDX-License-Identifier: BSD-2-Clause
+
+config PLATFORM_ANDES_AE350
+	bool
+	default y