In tightly embedded builds (--enable-embedded), we want the ability to
replace talloc with a very simple heap allocator to avoid the complexity
of talloc without modifying all our code that assumes talloc.
This will break the hierarchical notion of the allocator, but
libosmo{core,gsm,coding,codec} don't rely on that anyway.
Change-Id: Ie341034076f242a813f081919dd09d845775ad35
