FP6: Add support for IMG packages

Only touch the partitions that are updated via OTA
Skip modemst1 & modemst2 erase
Erase userdata & metadata instead of flashing a prebuilt image

Hence, we differ to the stock fastboot flash script with the following lines:

```
echo "Partition table..."
fastboot flash partition:0 gpt_both0.bin
fastboot flash partition:1 gpt_both1.bin
fastboot flash partition:2 gpt_both2.bin
fastboot flash partition:3 gpt_both3.bin
fastboot flash partition:4 gpt_both4.bin
fastboot flash partition:5 gpt_both5.bin

echo "Flash nCPU..."
fastboot flash storsec storsec.mbn

echo "Flash Apps..."
fastboot flash userdata userdata.img
fastboot flash metadata metadata.img
fastboot flash study study.tar

echo "6350 new images"
fastboot flash apdp apdp.mbn
fastboot flash logfs logfs_ufs_8mb.bin

echo "7635 new images"
fastboot flash toolsfv tools.fv
fastboot flash vm-persist vm-persist.img

echo "6490 new images"
fastboot erase modemst1
fastboot erase modemst2
```