When running sandbox, the following phases occur, each with different
malloc implementations or behaviors:

1) Dynamic linker execution, using the dynamic linker's own malloc()
implementation. This is fully functional.

2) After U-Boot's malloc symbol has been hooked into the GOT, but before
any U-Boot code has run. This phase is entirely non-functional, since
U-Boot's gd symbol is NULL and U-Boot's initf_malloc() and
mem_malloc_init() have not been called.

At least on Ubuntu Xenial, the dynamic linker does make both malloc() and
free() calls during this phase. Currently these free() calls crash since
they dereference gd, which is NULL.

U-Boot itself makes no use of malloc() during this phase.

3) U-Boot execution after gd is set and initf_malloc() has been called.
This is fully functional, albeit via a very simple malloc()
implementation.

4) U-Boot execution after mem_malloc_init() has been called. This is fully
functional with a complete malloc() implementation.

Further...