kernel: make MULTITHREADING promptless if single-thread not supported

If single thread builds are not supported by the
architecture, the MULTITHREADING option should be
prompt-less to block any modifications to it. We
also introduce an explicit ARCH-level Kconfig that
reflects whether the ARCH is capable of single-thread
Zephyr builds.

Signed-off-by: Ioannis Glaropoulos <Ioannis.Glaropoulos@nordicsemi.no>

arch/Kconfig

@@ -464,6 +464,9 @@ endmenu
 # Architecture Capabilities
 #
 
+config ARCH_HAS_SINGLE_THREAD_SUPPORT
+	bool
+
 config ARCH_HAS_TIMING_FUNCTIONS
 	bool
 

arch/arm/core/aarch32/Kconfig

@@ -9,6 +9,7 @@ config CPU_CORTEX_M
 	select ARCH_HAS_CUSTOM_SWAP_TO_MAIN
 	select HAS_CMSIS_CORE
 	select HAS_FLASH_LOAD_OFFSET
+	select ARCH_HAS_SINGLE_THREAD_SUPPORT
 	select ARCH_HAS_THREAD_ABORT
 	select ARCH_HAS_TRUSTED_EXECUTION if ARM_TRUSTZONE_M
 	select ARCH_HAS_STACK_PROTECTION if (ARM_MPU && !ARMV6_M_ARMV8_M_BASELINE) || CPU_CORTEX_M_HAS_SPLIM

kernel/Kconfig

@@ -10,7 +10,7 @@ module-str = kernel
 source "subsys/logging/Kconfig.template.log_config"
 
 config MULTITHREADING
-	bool "Multi-threading"
+	bool "Multi-threading" if ARCH_HAS_SINGLE_THREAD_SUPPORT
 	default y
 	help
 	  If disabled, only the main thread is available, so a main() function