discover/grub: Add blscfg command support to parse BootLoaderSpec files
The BootLoaderSpec (BLS) defines a file format for boot configurations,
so bootloaders can parse these files and create their boot menu entries
by using the information provided by them [0].
This allow to configure the boot items as drop-in files in a directory
instead of having to parse and modify a bootloader configuration file.
The GRUB 2 bootloader provides a blscfg command that parses these files
and creates menu entries using this information. Add support for it.