Makefile: Compile lib/utils sources separately for each platform

Currently, if same build directory is used to compile two different
platforms then lib/utils objects are shared for these platforms.

We will be having platform specific configs to enable/disable drivers
in lib/utils and select compile time options for lib/utils sources.
This means lib/utils sources will now be compiled in a platform
specific way.

To tackle above, we update top-level Makefile as follows:
1) Don't create libsbiutils.a anymore because this can't be shared
   between platforms.
2) Compile lib/utils sources separately for each platform.
3) Add comments showing which make rules are for lib/sbi, lib/utils,
   firmware, and platform sources.

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>