mckernel overlay: replace mcoverlayfs with a soft userspace overlay

mcoverlayfs has a high maintenance burden and does not work on rhel8's 4.18
kernel (while it works on vanilla 4.18...); instead of debugging this further
time is better spent making it independent from overlayfs.

Change-Id: I7454ae95b0fbb3373c256aa2fd83cdfec466c009

lib/include/list.h

@@ -1,7 +1,9 @@
 #ifndef _LINUX_LIST_H
 #define _LINUX_LIST_H
 
+#ifndef offsetof
 #define offsetof(TYPE, MEMBER) ((size_t) &((TYPE *)0)->MEMBER)
+#endif
 /**
  * container_of - cast a member of a structure out to the containing structure
  * @ptr:        the pointer to the member.