Install busybox On Debian/stretch as it's required for initramfs

fixes https://github.com/grml/grml-debootstrap/issues/263

Followup fix for 2708f441

Thanks: Patrick Schleizer <adrelanos@whonix.org> for spotting and reporting
Closes: https://github.com/grml/grml-debootstrap/issues/256

change from EFI to ESP for arm64 builds

instead of sometimes mixed with "${MNTPOINT}/boot/efi"

because no longer provided by Debian

to work around an issue by github actions
drop `-q` from modprobe for better debug output

because it will not always work

will not work in cases where /dev etc is still mounted inside the chroot

because the cleanup function gets run by the error handler and should
not be nested

https://github.com/grml/grml-debootstrap/issues/224#issuecomment-1779673194

because these are now covered by the new error handling method

only delete `$MNTPOINT` if such a folder actually exists

only delete `$MNTPOINT` if such a folder actually exists

No need to use both, `eend 1` and `bailout 1`.
In these cases, only `bailout 1` is sufficient.

because these are now covered by the error_handler