libutils: refactor the MDBG malloc functions

Replace the MDBG malloc functions and macros with a new function
__mdbg_alloc() handling all the memory allocations similar to
mem_alloc(). The old MDBG macros are replace by new macros calling
__mdbg_alloc().

mem_alloc(), mem_alloc_unlocked(), and mem_free() are updated to handle
configuration with and without MDBG, removing the all the ENABLE_MDBG
ifdefs in the C file.

Signed-off-by: Jens Wiklander <[email protected]>
Reviewed-by: Jerome Forissier <[email protected]>