New features:

* auto detect EFI boot and install EFI ESP (force with env USE_EFI=1)
* allow specifying bootloader by env BOOTLOADER (grub or syslinux)
* allow specifying disk label by env DISKLABEL (dos or gpt)

TODO:

* Make sure MBR is only overwritten on native installs.
* Do some more checks to prevent non standard/supported setup
  like GPT on BIOS installs.

TESTS run on ESXi 6.5:

Simple native install

* syslinux OK
* GRUB OK

Extended native install

* LVM OK
* RAID OK
* RAID+LVM OK

EFI install

* Default OK
* LVM OK see: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=592834
* RAID NOT SUPPORTED

Data install DOS

* Default OK
* LVM OK

Data install GPT

* Default OK
* LVM OK but: /lib/rc/sh/openrc-run.sh: line 273:
  can't create /sys/fs/cgroup/openrc/syslog/tasks: nonexistent directory