docs: Update documentation for kconfig support

We update all documentation files to:
1) Remove references to platform specific config.mk file since it is
   has been removed.
2) Add details about platform specific configs/defconfig and Kconfig
   files mandatory for each platform.
3) Add required packages in top-level README.md
4) Fix typo releated to object.mk in docs/platform/platform.md

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>