Refactor all CMake toolchains to reset CMAKE_INSTALL_PREFIX variable.
Add new build options for specifying tool-prefix and target-sysroot in cross-compiling build.
Add new IOS_SYSROOT variable to store the path to iOS sysroot and use it to set CMAKE_FIND_ROOT_PATH variable.
Now also generate pkg-config Urho3D.pc for iOS platform.
For Travis CI - simplify SDK installation logic. Use DESTDIR=~ to avoid sudo.