configs/beaglebone_qt5: increase size of ext4 filesystem image

In commit c6bca8ce, the
auto-calculation logic of the ext4 filesystem size was removed. The
default size of 60MB is too small for the beaglebone_qt5_defconfig,
should it should be increased.

Signed-off-by: Michael Trimarchi <michael@amarulasolutions.com>
Suggested-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>