Support redundant ESPs

[nickles95]

It would be nice for raid setups if clr-boot-manager could support copying kernels to ESP partitions on multiple disks automatically, so if one disk dies the system will just boot off the second one. You can work around this by mounting both ESP partitions and copying new kernels manually whenever they get updated, but it would be nice if this could happen automatically. The desired behavior would be the same as what proxmox-boot-tool does to support multiple ESP partitions: https://pve.proxmox.com/wiki/Host_Bootloader