qt: fix mysql module build

Closes #2371

Bug 2371 was a Qt compilation issue when MySQL support was enabled,
because the Qt configuration tests were looking at host MySQL
libraries.

Even though I couldn't reproduce the bug with MySQL libraries
installed on my host, there was clearly a problem since
-I/usr/include/mysql and -L/usr/lib/mysql were part of the compilation
flags when building those tests.

This was caused by the fact that /usr/bin/mysql_config was used
instead of $(STAGING_DIR)/usr/bin/mysql_config. We fix this by passing
the path to the correct mysql_config script using the -mysql_config Qt
./configure argument. Since this mysql_config script returns the
correct flags for libraries and headers path, the hardcoded -I and -L
flag in qt.mk are no longer needed.

With this in place, Qt is no longer compiled with -I/usr/include/mysql
and -L/usr/lib/mysql, which presumably should fix the problem reported
in bug 2371.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>