rpi: update to latest kernel; fix rootfs size

Updating Buildroot forced the kernel update, but it seems to be
harmless. The rootfs max size was also fixed to be 128 MB like specified in
fwtool.config in the comments.
Add patch to dtach to support redirecting the erl prompt
nerves.mk: alternative to sourcing nerves-env.sh

For Nerves configurations that pull in Nerves as a subdirectory to the
project, this file can be included in a Makefile to achieve similar
results as sourcing nerves-env.sh.
Bump buildroot now that they're getting close to release

Clean up dtach patch
Remove unionfs support from defconfigs

Unionfs has been buggy on x86 and it's unclear whether there are issues
on the other targets. Now that relsync doesn't require it, remove it
from being a default.
Fix up Raspberry Pi config.txt handling
elixir: bump version to 0.13.3
fwup: new package
Bump buildroot version to 2014.05 release, libarchive patch

The libarchive patch is needed to build fwup.
fwup: bump to version v0.0.1
Step 1 of migration from fwtool to fwup

Raspberry Pi and BBB configs should work now.
Minor fixes to fwup support on bbb/rpi
Update the ag150 and alix to use fwup
Remove fwtool
elixir: bump to version 0.14
fwup: bump to v0.0.3
Update mtools dependencies

host-mtools is used by syslinux. fwup provides fat file system support
for rpi
Fix comment regarding fwup