Update mkqt5bld.py script

Add options to:
- provide prebuilt icu as an argument
- provide prefix as an argument (-prefix)
- provide runtime path as an argument (-R)

Change-Id: I116e5044d7c8c09131dd8c38d771ae4185455461
Reviewed-by: Sergio Ahumada <sergio.ahumada@digia.com>