util: add atterror2str function helper

This function converts an ATT error in a human readable string.
Add characteristics command

With this command we can iterate over all characteristics of specific
service. The result will be handled at get_characteristic_callback.
Add characteristic storage buffer

This commit creates a service_info struct and a char_info struct. The
service info struct stores service information and a buffer of char_info
structs that will store information about characteristics of that
specific service. In the future the char_info struct will be extended to
store a buffer with information about the descriptors of that specific
characteristic.

The characteristics buffers grow on demand. We allocate a 8-entries
buffer per service and grow it with realloc if needed.