Disabling IRQs during malloc() provides mutually exclusive access and even is
safe from IRQ context, but is suboptimal for real time scenarios. Instead, the
implementation is changed to use a mutex to provide mutually exclusive access.
As a result, calls to malloc() and free() from IRQ context no longer is
possible. But this this is a really horrible idea to begin with, the impact
should be minimal and the improved real time properties of the system should
make it a good trade-off. An assert() is added to allow easy detection of
regressions and, hence, aid users to fix their code.
Split out Gunar Schorcht's clever approach to provide thread safe malloc for
AVR into a system module and make AVR depend on this. This allows other
platforms to also use this.
