cspec/c: provide NUM_DOMAINS build override option

Setting the environment variable INPUT_NUM_DOMAINS will cause the
build to override the KernelNumDomains setting in the config file with
the provided setting.

Signed-off-by: Gerwin Klein <gerwin.klein@proofcraft.systems>

spec/cspec/c/kernel.mk

@@ -108,6 +108,10 @@ ${OVERLAY}: ${DEFAULT_OVERLAY}
 	@cp $< $@
 endif
 
+ifdef INPUT_NUM_DOMAINS
+KERNEL_CMAKE_EXTRA_OPTIONS += -DKernelNumDomains=${INPUT_NUM_DOMAINS}
+endif
+
 # Initialize the CMake build. We purge the build directory and start again
 # whenever any of the kernel sources change, so that we can reliably pick up
 # changes to the build config.