Draft: Add support for flashing to devices with Tow-Boot installed in shared storage mode

https://tow-boot.org is a pre-configured u-boot which will be used on the PPP (PinePhone Pro) to boot. Developer and Explorer Edition devices of this device don't have a dedicated boot storage so Tow-Boot has to be installed on the eMMC, on which the OS will also reside. Thus we should make sure we don't overwrite an already installed Tow-Boot or we might risk bricking people's devices

This uses https://sr.ht/~martijnbraam/findtow/ to detect if Tow-Boot is installed on the specified storage.

Note that this is as of yet untested on the internal eMMC of the PPP and requires https://gitlab.com/postmarketOS/pmaports/-/merge_requests/2870. Also after installing Tow-Boot the GPT partition table needs to be extended to cover the whole disk: sgdisk -e /path/to/device This is now done in pmbootstrap itself