added buildroot as submodule

buildroot set to tag 2012.05 where it was at latest
upstream Cirros update.
update buildroot and its config to current master
fix compilation with latest buildroot
automate choice of number of make workers for buildroot build
update buildroot config to have support for C++

This is made to be a separate commit because it adds
quite much noise to the conf file delta and having this
done, subsequent modifications can be kept clean.'
added some utilities to buildroot config

- nmap
- strace
- iptables
- s/screen/tmux/
- busybox bumped 1.20 -> 1.22
add support for nfs and samba

- both server and client
- nfs: both v3 and v4
update README to reflect changes
fix typo in patch queue
add notes on customization to README
buildroot: remove obsolete sudo-install-sudoers-so.patch

The issue the patch addresses is not present in current buildroot version

Fixes #1
fix nfs issues

- nfsv4 support was broken due to missing dependencies
- nfsd.ko was missing from image

Fixes #2
bin/bundle: rework image size calculation

Instead of relying on a pre-determined image size,
image size is calculated as the size of the rootfs
tarball plus extra size.

Reflecting the change in semantics, earlier -s/--size option is
replaced with -x/--xsize.

Fixes #3
fix urls for kernel and grub
buildroot: add htop
buildroot: add rpcbind

Fixes #4
start v4 capable nfsd service on boot
fix remote execution issues due to too restictive dropbear $PATH
add some Ubuntu compat shims
Allow default file creation with 755 permission

It's necessary that files by default are created with 755 permission -
similar to Ubuntu. Currently they are created with 700 permission.