blockdev: add bootable property to find ESP partition

[HuijingHei]

As Fedora IoT on the Raspberry Pi 3 is using MBR by default, we need to find ESP partition via bootable.
See coreos/bootupd#1019

[gemini-code-assist]

Code Review

This pull request correctly adds support for identifying bootable partitions, which is necessary for MBR partition tables. The changes to the data structures and the new helper functions are well-implemented. I've included one suggestion to improve the new test case to make it more robust and idiomatic.

[cgwalters]

Nothing wrong with adding a bootable property, but I don't think that's how we should find the ESP - per Wikipedia

The globally unique identifier (GUID) for the EFI system partition in the GUID Partition Table (GPT) scheme is C12A7328-F81F-11D2-BA4B-00A0C93EC93B, while its ID in the master boot record (MBR) partition-table scheme is 0xEF.

So we should search for 0xEF instead.

[HuijingHei]

Nothing wrong with adding a bootable property, but I don't think that's how we should find the ESP - per Wikipedia

The globally unique identifier (GUID) for the EFI system partition in the GUID Partition Table (GPT) scheme is C12A7328-F81F-11D2-BA4B-00A0C93EC93B, while its ID in the master boot record (MBR) partition-table scheme is 0xEF.

So we should search for 0xEF instead.

Agree, find bootable looks more easier, and 0xEF is more accurate, will add it later, thank you for the pointer!

[cgwalters]

So we should define a const ESP_ID_MBR and use that both in bootupd and bootc, and move the esp_in() code down to blockdev.rs and have it detect MBR vs GPT.