modules: rename packages/kernel_modules to packages/lkmc

Then inside, split packages/lkmc into kernel_modules and userland,
to keep userland out of the kernel_modules parent path, which makes no
sense.

Copy built modules and userland to the output rootfs overlay.

Document Linux distro tradeoffs.

packages/lkmc/userland/openmp.c

@@ -0,0 +1,19 @@
+/* https://github.com/cirosantilli/linux-kernel-module-cheat#openmp */
+
+#include <omp.h>
+#include <stdio.h>
+#include <stdlib.h>
+
+int main () {
+	int nthreads, tid;
+#pragma omp parallel private(nthreads, tid)
+	{
+		tid = omp_get_thread_num();
+		printf("Hello World from thread = %d\n", tid);
+		if (tid == 0) {
+			nthreads = omp_get_num_threads();
+			printf("Number of threads = %d\n", nthreads);
+		}
+	}
+	return EXIT_SUCCESS;
+}