Refactor and clean up Grub2 setup

[davidcassany]

This is an issue (probably epic) that encompasses few Grub2 related changes. The main reason I tried to put them together is because bootloader changes are sensitive and we should avoid regular changes in that area. So probably we can encompass those all together.

• Dont run tests if only Dockerfile has changed elemental-cli#151 (done, however we keep the issue opened until SLE includes loopback and squashfs modules for the grub2 EFI signed images)
• Influence kernel parameters at install time elemental-toolkit#1773 (nice to have, no strict requirement for the RC)
• Keep working dir after a chroot call elemental-cli#324 (currently affecting only BIOS firmware installs, so not that relevant, however still a low hanging fruit)
• Failed preparing ISO's root tree: exit status 5 for v5.2 elemental-cli#325 (nice to have, but not for now, no changes on the boot procedure before RC)

[kkaempf]

Closing this issue as the main part is done. The remaining issues stay open but are not in scope right now.