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/init_env_poweroff.c

@@ -0,0 +1,26 @@
+#define _XOPEN_SOURCE 700
+#include <stdio.h>
+#include <sys/reboot.h>
+#include <unistd.h>
+
+int main(int argc, char **argv)
+{
+	int i;
+
+	puts("args:");
+	for (i = 0; i < argc; ++i)
+		puts(argv[i]);
+	puts("");
+
+	puts("env:");
+	extern char **environ;
+	char **env = environ;
+	while (*env) {
+		printf("%s\n", *env);
+		env++;
+	}
+	puts("");
+
+	/* Poweroff. */
+	reboot(RB_POWER_OFF);
+}