Decouple caching of iOS/OSX SDK path and version in sdk.prf

Combining them could lead to intermediate builds having cached the
path, but not the version, resulting in later version checks failing.

Change-Id: Ia10f4268ce7b9e82c81627970236d68c00b80391
Reviewed-by: Richard Moe Gustavsen <richard.gustavsen@theqtcompany.com>
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@digia.com>