This is 'debranding' in the sense that live-build defaults are for people
building images with a config from scratch, live-build should not
have the options that we use for the official prebuilt debian-live
images as debian-defaults for all images and force that upon everyone
(as defaults). Rather, a 'lb config' should produce a as vanilla as
possible configuration tree.

Dropping powerpc bootloader support (yaboot), there is nobody actively maintaining this in live-build.

Allowing to use parted its automatic offset handling at the beginning of a disk, rather than to harcode it to first block (Closes: #647607).

Dropping theme package support, themes are either in the config tree or the embedded one from live-build is used.

Adding bootrap_archive-keys to establish secure trust-chain on top of debian-keyring for derivatives.

Correcting vshift in syslinux themes to correct display bug with overlong lines, thanks to Thanatermesis <thanatermesis@gmail.com>.

Switching default binary filesystem for hdd images to fat32 to be more robust on todays flash devices.