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/sched_getaffinity.c

@@ -0,0 +1,42 @@
+/* https://github.com/cirosantilli/linux-kernel-module-cheat#gdb-step-debug-multicore */
+
+#define _GNU_SOURCE
+#include <assert.h>
+#include <sched.h>
+#include <stdbool.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <unistd.h>
+
+void print_affinity() {
+    cpu_set_t mask;
+    long nproc, i;
+
+    if (sched_getaffinity(0, sizeof(cpu_set_t), &mask) == -1) {
+        perror("sched_getaffinity");
+        assert(false);
+    } else {
+        nproc = sysconf(_SC_NPROCESSORS_ONLN);
+        printf("sched_getaffinity = ");
+        for (i = 0; i < nproc; i++) {
+            printf("%d ", CPU_ISSET(i, &mask));
+        }
+        printf("\n");
+    }
+}
+
+int main(void) {
+    cpu_set_t mask;
+
+    print_affinity();
+    printf("sched_getcpu = %d\n", sched_getcpu());
+    CPU_ZERO(&mask);
+    CPU_SET(0, &mask);
+    if (sched_setaffinity(0, sizeof(cpu_set_t), &mask) == -1) {
+        perror("sched_setaffinity");
+        assert(false);
+    }
+    print_affinity();
+    printf("sched_getcpu = %d\n", sched_getcpu());
+    return EXIT_SUCCESS;
+}