buflib: Add buflib_context_relocate().

This function relocates a buflib back buffer, updating pointers in struct
buflib_context. It does not move any data by itself.

The intended use-case is buflib-on-buflib, where a buflib back buffer is
allocated with buflib and attempted to be moved. The move_callback() can call
this and return BUFLIB_CB_OK on success. No move_callback() is called for the
subordinate buflib buffer, therefore it must not contain non-movable
allocations. The caller is generally responsible moving the data and all its
implications.

Change-Id: I869219f9cff786a172c9e917a5f34470073892e6

firmware/include/buflib.h

@@ -157,6 +157,21 @@ size_t buflib_available(struct buflib_context *ctx);
  */
 size_t buflib_allocatable(struct buflib_context *ctx);
 
+/**
+ * Relocates the fields in *ctx to the new buffer position pointed to by buf.
+ * This does _not_ move any data but updates the pointers. The data has
+ * to be moved afterwards manually and only if this function returned true.
+ *
+ * This is intended to be called from within a move_callback(), for
+ * buflib-on-buflib scenarios (i.e. a new buflib instance backed by a buffer
+ * that was allocated by another buflib instance). Be aware that if the parent
+ * move_callback() moves the underlying buffer _no_ move_callback() of the
+ * underlying buffer are called.
+ *
+ * Returns true of the relocation was successful. If it returns false no
+ * change to *ctx was made.
+ */
+bool buflib_context_relocate(struct buflib_context *ctx, void *buf);
 
 /**
  * Allocates memory from buflib's memory pool