add basic configuration for 'lb build':

- build an amd64 live system (Debian 10 "Buster")
 - don't install 'recommended' packages
 - enable contrib/non-free sections (required for firmwares)
 - enable stable-backports,security,stable-updates repositories
 - don't include source packages in the build (save space)
 - generate iso-hybrid images suitable for boot from USB
 - enable 'installer' mode allowing to install the live system to a fixed disk
 - install packages required for HTTPS APT sources
 - include binary firwares on the iso image
 - included memtest86+ and win32-loaded on the bootable media

auto/clean

@@ -0,0 +1,11 @@
+#!/bin/sh
+
+lb clean noauto "$@"
+rm -f binary.log prepare.log
+rm -rf	.build/ \
+		config/binary \
+		config/bootstrap \
+		config/build \
+		config/chroot \
+		config/common \
+		config/source