nixos: increase allocated memory for gnome tests. Closes #4651
bcache-tools: don't use probe-bcache and bcache-register

This allows us not to include any additional binaries for bcache
support in initrd; just an udev rule.
stage-1: add bcache support

This adds necessary udev rule into initrd to support autodetecting
bcache partitions in stage-1. It also fixes 'sh' path for the new
rule to work.
r-RcmdrPlugin.ROC: disable broken build
haskell-graphviz: disable test suite because it runs way too long

The test suite runs for ages, i.e. 1+ hours. That seems a little excessive.
Merge pull request #4742 from abbradar/bcache-stage1

Support bcache volumes in initrd
nixos: iso-image: use syslinux bootloader for USB booting support

This changes the bootloader for iso generation from Grub to
syslinux. In addition this adds USB booting support, so that
"dd" can be used to burn the generated ISO to USB thumbdrives
instead of needing applications like UnetBootin.
nixos: installation: make installation ISOs USB bootable
mysql: use another download url, cdn.mysql.com is slow after 30%
Update SBCL to 1.2.5
WIP: nix-prefetch-git: bring back git index (with leaveDotGit)

FIXME: Still not deterministic!?

The git index contains timestamps. So far we have removed the index to
have deterministic output. But removing the index makes the tree
"dirty". Not good for build systems that use "git describe --dirty" to
generate a version string.

This patch changes nix-prefech-git (and fetchgit) to run git with
libfaketime (and time set to UNIX epoch). This way we can keep the index
and still have deterministic output.

Implementation comments:
* Use bash arrays in the wrappers to be able to pass FAKETIME
  environment variable, which contains whitespace, to the
  nix-prefetch-git wrapper.
* nix-prefetch-git is part of the nix-prefetch-scripts package. Only
  the *-git script is modified by this patch, as I don't know how fake
  time would affect the other fetchers.