add BOOT_APPEND to GRUB_CMDLINE_LINUX_DEFAULT

it was broken since commit cbf40a49324ae7feb468e528a7845465304faaff
Do not hardcode squeeze as Debian/stable

This will be confusing once Debian/wheezy is the current stable version
of Debian, so let's avoid making it more confusing than necessary. :)
Set wheezy as the new default release [Closes: #688234]
docs: fix outdated default mirror reference
docs: clarify AUTOINSTALL is currently supported on Grml live systems only

It could be supported by any live system, but currently only Grml
provides it, so clarify that.

Thanks: Evgeni Golov <evgeni@grml.org> for review + improving
docs: mention VM image and dialog based frontend support

Thanks: Evgeni Golov <evgeni@grml.org> for review and improving
Release new version 0.54
Merge branch 'mika/wheezy_release'