Because Huawei, fix #400

Fix #399

In previous versions, magiskinit will not early mount if /sepolicy is detected. However on OP5/5T latest betas, the devices are fully trebelized,
but for some reason the file /sepolicy still exists, making magiskinit think it is NOT a treble device and doesn't work properly.

So to properly fix this issue, I will have to use the "official" way - check fstab in device trees. Any block mentioned in the fstab in device trees
are supposed to be early mounted. Currently magiskinit will only mount system and vendor even if other partitions exists in the dtb fstab, since other
partitions are not used to construct sepolicy (currently).

These changes can also fix #373, since we dynamically detect PARTNAME from device trees.

Reduce even more size for static binaries