Bug#1031929: support configuration of debootstrap binary

[Patrick Schleizer <adrelanos@whonix.org>]

Package: live-build
Severity: wishlist

Dear maintainer,

currently scripts/build/bootstrap_debootstrap hardcodes `debootstrap`.

debootstrap ${DEBOOTSTRAP_OPTIONS} "${LB_PARENT_DISTRIBUTION_CHROOT}" 
chroot "${LB_PARENT_MIRROR_BOOTSTRAP}" ${DEBOOTSTRAP_SCRIPT}

There are however other implementations of `debootstrap` such as I am 
aware of:

- `cdebootstrap`,

- `mmdebstrap` (which is amazing because it allows bootstraping from 
Debian "normal" repository + security repository, apt-cacher-ng APT 
caching compatible, compatible with https TLS, onion, much faster than 
`debootstrap`, and more)

Could you please add an option debootstrap programs? `lb config` could 
provide a config option such as:

> --debootstrap-program program

> tells live build to use an debootstrap program.

Kind regards,
Patrick