Update feature paths on assignment to QMAKE_PLATFORM

Instead of after parsing the makespecs, were we assumed QMAKE_PLATFORM
had been set by the makespec and did an explicit update.

Allows loading platform specific features from within a makespec after
updating QMAKE_PLATFORM.

Change-Id: I0eb3b7fb88ce48b00a384850c5d87223c06234d7
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@digia.com>
(cherry picked from qtbase/4dd29248cd43ff730f7432b51a443b72fad3523d)