qt4: fix QMAKE_QT_CONFIG being overwritten with empty value

qt4: fix QMAKE_QT_CONFIG being overwritten with empty value

QMAKE_QT_CONFIG is set by the linux-oe-g++ mkspec but it is overwritten
with an empty value read from .qmake.cache. Avoid this by first checking
if the value from .qmake.cache is not empty before assigning it to
QMAKE_QT_CONFIG.

This allows variables from qconfig.pri such as QT_ARCH, QT_CONFIG,
QT_VERSION, etc. to be queried by qmake projects.

Signed-off-by: Jonathan Liu <net147@gmail.com>
Signed-off-by: Saul Wold <sgw@linux.intel.com>