Add ability to configure some important coreboot config settings

Nobody has claimed this yet.

**Is your feature request related to a problem? Please describe.**
See https://github.com/Dasharo/dasharo-issues/issues/285 (coreboot allows configuring battery charging limits both from the BIOS as well as the OS); this used to work in Heads 2.1 for me as well on my NV41.

See https://github.com/Nitrokey/heads/issues/35 (Setting battery charging limits worked in NK Heads 2.1, but not anymore in NK Heads 2.4 and 2.4.1). In that issue @daringer says that it's an upstream issue with Heads.

**Describe the solution you'd like**
Allow configuring battery charging limits (and possibly other coreboot configurable settings) from within Heads or the OS.

**Describe alternatives you've considered**
Unfortunately right now there doesn't seem to be an alternative other than removing Heads and using Coreboot (or going back to Heads 2.1). The usually suggested solutions online, e.g. `tlp` or `smbios-utils` all require that the firmware make the relevant system files, usually `/sys/class/power_supply/BAT0/charge_control_start_threshold` and `/sys/class/power_supply/BAT0/charge_control_end_threshold`, available to the OS, as that is how those utilities set the new limits.

**Additional context**
The focus is on the battery charging limits, as that's not just a convenience feature, but also important to help preserve battery life for laptop users.

Start by reviewing the linked Dasharo and Nitrokey issues and the existing Heads 2.1 behavior described here. Determine how battery charging limits could be exposed from Heads or the OS, with /sys/class/power_supply/BAT0/charge_control_start_threshold and charge_control_end_threshold as the stated target; done means users can configure those limits again.

Written by the indexing model from the issue text.