oniro-image-base-tests: bump rootfs size to 2500M

Building the base test image with debug mode enabled returns the below
error.

```
ERROR: File system image of partition / is larger (2269180 kB) than its
allowed size 2097152 kB.
```

Increasing rootfs size to 2500M solves the issue.

Signed-off-by: Chase Qi <chase.qi@linaro.org>