rl_helper: add history command management

We need a growable buffer which will store a list of command strings. So
when we press up or down keys, our line buffer is replaced with old
commands that we typed.

This also fix a possible bug, cause we pass a copy of line buffer to
line processing callback. So we don't will have problems if we change
it's value (eg strtok).
Add Write Characteristic commands

We are adding a generic write characteristic command with write_type as
parameter, so we are able to implement Write Command and Write Request
GATT operations.