qt5base: adjust linux-buildroot-g++ qmake.conf

In order for qmake to generate correct moc command lines, we need to
make sure INCLUDEPATH contains $(STAGING_DIR)/usr/include. To do so,
we add a new line in our custom qmake.conf.

Also, in order to clearly identify the Buildroot specific variables
from the qmake standard variables, we prefix all the custom Buildroot
variables by BUILDROOT_ in qmake.conf.

This solves compilation issues of qt5svg such as "Undefined
interface", due to the fact that moc isn't passed a -I path pointing
to the main header directory.

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