lkmc_many_files: create package to test a rootfs with many files

Automatically pick up packages from under packages/ into BR2_EXTERNAL.

Move many subdirectory READMEs into the toplevel and link to toplevel from those subreadmes instead.

packages/sample_package/sample_package.c

@@ -0,0 +1,7 @@
+#include <stdio.h>
+#include <stdlib.h>
+
+int main(void) {
+	puts("hello sample_package");
+	return EXIT_SUCCESS;
+}