Kconfig: don't fall back on CONFIG_TIGERLAKE

There's really no reason to fall back on a default "platform" and pick
up a bunch of other, totally random settings.

Now the following command fails immediately with a useful `platform not
defined` error message instead of stopping with a cryptic
`platform/trace/trace.h: No such file or directory` after successfully
compiling half the .c files in a Frankenstein CONFIG_uration.

```
  west build -p --board mimx8mm_evk_a53 modules/audio/sof/app
```

More context in #7192.

Signed-off-by: Marc Herbert <marc.herbert@intel.com>

src/arch/xtensa/CMakeLists.txt

@@ -22,6 +22,8 @@ elseif(CONFIG_MT8188)
         set(platform_folder mt8188)
 elseif(CONFIG_MT8195)
 	set(platform_folder mt8195)
+else()
+	message(FATAL_ERROR "Platform not defined, check your Kconfiguration?")
 endif()
 
 set(fw_name ${CONFIG_RIMAGE_SIGNING_SCHEMA})

src/platform/Kconfig

@@ -5,7 +5,9 @@ menu "Platform"
 choice
 	prompt "Platform"
 	default ZEPHYR_POSIX if ARCH_POSIX
-	default TIGERLAKE
+	# It's not really 'optional' but no value is much less confusing
+	# than falling back on a totally random value.
+	optional
 
 config TIGERLAKE
 	bool "Build for Tigerlake"

zephyr/CMakeLists.txt

@@ -363,6 +363,9 @@ zephyr_library_sources_ifdef(CONFIG_LIBRARY
 	${SOF_DRIVERS_PATH}/host/ipc.c
 )
 
+if(NOT DEFINED PLATFORM)
+	message(FATAL_ERROR "PLATFORM is not defined, check your Kconfiguration?")
+endif()
 zephyr_include_directories(${SOF_PLATFORM_PATH}/${PLATFORM}/include)
 
 # Mandatory Files used on all platforms.