nativesdk-qtbase: Add CMake toolchain configuration

This allow out of box use of CMake in a SDK; this includes the
subscript to properly set the host binaries path.

Signed-off-by: Otavio Salvador <otavio@ossystems.com.br>
nativesdk-qtbase: Fix path for mkspecs in SDK

The mkspecs file needs to use the QT_DIR_NAME variable to
have a proper path to the files, fix it.

Signed-off-by: Otavio Salvador <otavio@ossystems.com.br>
qtsystems: Change branch from stable to 5.3

Stable branch was removed on mainline and now 5.3
is used.

Signed-off-by: Diego Sueiro <diego.sueiro@gmail.com>
Signed-off-by: Otavio Salvador <otavio@ossystems.com.br>