CMake: Add KernelIsMCS option

This switches between master and mcs configurations.
This also adds a build system variable KernelPlatformSupportsMCS that
can be used to error on platforms that don't support MCS due to
unimplemented functionality.

config.cmake

@@ -14,8 +14,17 @@ cmake_minimum_required(VERSION 3.7.2)
 
 set(configure_string "${config_configure_string}")
 
-# Set kernel branch
-config_set(KernelIsMaster KERNEL_MASTER ON)
+config_option(
+    KernelIsMCS KERNEL_MCS "Use the MCS kernel configuration, which is not verified."
+    DEFAULT OFF
+)
+
+# Error for unsupported MCS platforms
+if(KernelIsMCS AND (NOT KernelPlatformSupportsMCS))
+    message(
+        FATAL_ERROR "KernelIsMCS selected, but platform: ${KernelPlatform} does not support it."
+    )
+endif()
 
 # Proof based configuration variables
 set(CSPEC_DIR "." CACHE PATH "")

configs/seL4Config.cmake

@@ -146,6 +146,10 @@ unset(KernelArmMachFeatureModifiers CACHE)
 unset(KernelArmCPU CACHE)
 unset(KernelArmArmV CACHE)
 
+# Blacklist platforms without MCS support
+unset(KernelPlatformSupportsMCS CACHE)
+set(KernelPlatformSupportsMCS ON)
+
 file(GLOB result ${CMAKE_CURRENT_LIST_DIR}/../src/plat/*/config.cmake)
 list(SORT result)
 
@@ -167,6 +171,7 @@ config_set(KernelArchArmV6 ARCH_ARM_V6 "${KernelArchArmV6}")
 config_set(KernelArchArmV7a ARCH_ARM_V7A "${KernelArchArmV7a}")
 config_set(KernelArchArmV7ve ARCH_ARM_V7VE "${KernelArchArmV7ve}")
 config_set(KernelArchArmV8a ARCH_ARM_V8A "${KernelArchArmV8a}")
+set(KernelPlatformSupportsMCS "${KernelPlatformSupportsMCS}" CACHE INTERNAL "" FORCE)
 
 # Check for v7ve before v7a as v7ve is a superset and we want to set the
 # actual armv to that, but leave armv7a config enabled for anything that