nokia-n900: add PMOS_FORCE_PARTITION_RESIZE

Resize the partition when using a pre-built image. We didn't notice that this was missing, because N900 images are new and we only tested from pmbootstrap install --sdcard, which already uses the full SD card size.

Fixes #1034 (closed).

I verified that it works, and it's trivial. Merging after CI passes and backporting to v21.03.