[all] look into fstab.$DEVICE if recovery.fstab doesn't have needed paths

This won't work as value of HYBRIS_BOOT_PART is not empty while compiling for hammerhead, it's just invalid.
Here is the make output without my fix for hammerhead :

```
hybris/hybris-boot/Android.mk:48: ********************* /boot should live on emmc
hybris/hybris-boot/Android.mk:49: ********************* /data should live on ext4
```