systemboot Implement localboot via BLS

This was brought up by @davide125

See https://systemd.io/BOOT_LOADER_SPECIFICATION

Jan 21 '19 12:01 tfg13

Considering implementing this at the coreboot/LB hackathon this week

Jun 02 '19 16:06 tfg13

RHEL 8 (and probably CentOS8) uses this by default and populates loader/entries/*.conf files inside the efi/boot partition.

I think it is nicer to parse those files, as they follow a specification, instead of parsing grub configs. I only intend to implement the non-EFI parts of the spec (called Type#1). The other part is EFI specific and requires loading an EFI PE binary, which is out of scope for now. (Although it is nice for things like signing/secureboot as everything is in a single file (kernel, initrd, commandline, metadata))

Need to discuss with @insomniacslk and @hugelgupf how we want to organize the multiple localboots.

Jun 02 '19 16:06 tfg13

I am going to open this PR against u-root as soon as the move is done, I don't want to delay this any longer.

Jun 06 '19 08:06 tfg13