Optional ESP bootloader installation

[vjr]

Distribution (run cat /etc/os-release):

See issue elementary/installer#662
and PR elementary/installer#675

Related Application and/or Package Version (run apt policy $PACKAGE NAME):

elementary OS installer.

Issue/Bug Description:

Related to the linked issue and PR this is a request to make bootloader installation optional for use cases where people are dual/multi booting different OS and do not want a distinst based installation overwrite their preferred bootloader setup.

Steps to reproduce (if you know):

Currently running the elementary OS (v8 early access) installer with UI allows to skip bootloader installation but distinst reports error (paraphrasing) "EFI bootloader not specified".

Expected behavior:

Installation should success without bootloader if the user so chooses.

Other Notes:
NA

[nroach44]

As someone very technical installing PopOS for someone non-technical, I'm honestly horrified that the default multi-boot (with Windows) path is "just MAKE A SECOND EFI BOOT PARTITION".

There's only meant to be one partition per disk. Fair game if you have multiple disks, but that's a stupid assumption.

The next logical option is to resize the partition, but wait, you need to shuffle every partition after the ESP (so you know, your whole Windows install) back to make space. That's stupid, slow, and risky.

So please please please just make a small kexec kernel+initramfs as a bootloader (see petitboot) so the OS install can live entirely on it's own partition and not clutter up the ESP.