LLDB: Fix command reading loop

It could entirely miss commands sent by the engine.
The loop was waiting for something to come into stdin, but always only
read a single line from it, although it possibly already contained
multiple lines.
The patch simplifies the loop by removing the select() call entirely
and solely using sys.stdin.readline() which blocks anyhow.

Change-Id: Iff12aa051bb6e11c5ba30081d7c9d4e98940e231
Reviewed-by: hjk <hjk121@nokiamail.com>