Add a config to use the custom _sbrk function, defined by a user.
It is possible that an application doesn't want to use the entire
remaining RAM for the heap.
Signed-off-by: Dawid Niedzwiecki <dawidn@google.com>
This commit relocates the newlib libc configurations under the top-
level libc Kconfig to a new Kconfig under `lib/libc/newlib` for
improved organisation of the libc configurations.
Signed-off-by: Stephanos Ioannidis <stephanos.ioannidis@nordicsemi.no>