buildroot

qt: use $(TARGET_MAKE_ENV) when calling $(MAKE)

We drop the usage of the QT_CONFIGURE_ENV variable, because it isn't
defined anywhere, so it's always empty.

Signed-off-by: Gustavo Zacarias <gustavo.zacarias@free-electrons.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>