Ubuntu – Ubuntu 17.10 corrupted bios Lenovo with USB boot device

You wrote:

It looks like the bootmanager should "boot to the BIOS settings" or something like that, but... HOW?

If you're saying that GRUB has a menu entry with that label, try using it. Many EFI boot managers (I think including GRUB) now offer such an option. The option doesn't work on all EFIs, though, so it's conceivable that GRUB has such an option but it doesn't work on your computer.

There's an option in Windows to reboot into the setup utility. This Acer page describes how to access it, but I'm not sure offhand if the instructions are specific to Acer's configuration or if they apply universally. I know I've seen a "generic" description somewhere, but I don't happen to have the URL bookmarked.

Another option is to use my rEFInd boot manager. It definitely offers an option to reboot into the firmware setup utility, although if rEFInd thinks the firmware doesn't support it, the option won't appear in the menu. (The option appears as an icon of a computer chip on the second row of icons.) You can try rEFInd temporarily by creating a CD-R or USB flash drive; ready-made images for these are available on the rEFInd downloads page. That could be a chicken-and-egg scenario, though, since you'll probably need to get to the computer's built-in boot manager to boot from a CD-R or USB drive. Thus, you may need to go all-in and install rEFInd to your hard disk to try it. (If you don't like it, you can always use efibootmgr to disable rEFInd. Type efibootmgr to see the current boot options, then change the boot order with efibootmgr -o, as in efibootmgr -o 2,0,7 to boot options 2, 0, and 7, in that order of preference.)

Once you're into the setup utility, look for options called fast boot, USB support, or variants of those terms. It's conceivable that installing Windows somehow changed the firmware defaults to speed up the boot process, and in doing so disabled the key to access the firmware at boot time. Adjusting these options back to their defaults should re-enable the feature you want, at the cost of a second or two of extra boot time.

If you are able to boot in Windows the BIOS can be re-flashed with the Y50-70 Laptop BIOS Update If you are not able to boot then you will not be able to run the BIOS update. You will have to send the laptop to Lenovo to re-flash the BIOS.

A clip From Phoronix - Ubuntu 17.10 Temporarily Pulled Due To A BIOS Corrupting Problem:

Should your BIOS be corrupted, you may need to replace your motherboard if there is not a removable flash chip. There are some reports that resetting the BIOS does work, but it's too early to know if that works for everyone. This issue has been confirmed for several different lines of Lenovo laptops including the Yoga and IdeaPad products. There is also the reports of it affecting a few Acer, Toshiba and Dell laptops.

When the BIOS is corrupted by the Intel SPI kernel driver, the effects range from being unable to save BIOS settings to no longer being able to boot from USB devices.

The Intel SPI kernel driver is responsible for reading/writing to SPI serial flash. Due to the SPI serial flash holding the BIOS and other platform specific data, Intel's driver is supposed to make the contents read-only, but clearly something is going awry with the driver in 17.10.