mediatek: increase size of the sdcard image to 512 MiB

mediatek: increase size of the sdcard image to 512 MiB

Increasing the size of the rootfs_data filesystem has become a ever
repeating discussion and seems to be the most important thing for
users of the MediaTek-based BananaPi boards.

Using the whole remaining size of a microSD or the eMMC for rootfs_data
doesn't make sense for many reasons, but neither does the current
default of 104 MiB for the 'rootfs' partition size.

Increase the 'rootfs' partition size to 448 MiB which will result in
the sdcard image being exactly 512 MiB. Finding a microSD card smaller
than 512 MiB and still working could anyway be difficult in 2024.

That will allow users to install even bloatware written in Go or other
space-hungry languages while still leaving most of the space unallocated
for additional partitions or volumes to be used for persistent user
data.

Signed-off-by: Daniel Golle <daniel@makrotopia.org>