Add support for picking from multiple crosscompilers

The buildroot toolchain adds a symlink to gcc, so we detect
multiple crosscompilers when we check. This logic prunes the list to one
or produces an error if no compiler was found.
erlinit.mk: pull in graceful shutdown
ag150: enable PM, multi-core, and RTC
ag150, alix: Fix partition overlap
elixir: bump version to 0.12.5
Refactor busybox configuration to common location
Make naming of post-build scripts consistent

The bbb had the old naming of the script. Now they should be the same.
Compile Elixir by default

Due to our use of Erlang releases, Elixir won't be copied to the target
if the user doesn't use it, so it's free to build as part of the SDK.
README: add information about non-bbb platforms
Pull in mtools updates from upstream BR
Add unionfs patch to buildroot

This is needed to make unionfs work on the Raspberry Pi due to the
_SITE_METHOD issue with Buildroot. It is a minor improvement for the
other platforms since the download URLs and unionfs version are now
contained in one file rather than multiple config files.
bbb: convert bbb configs to use unionfs option
raspberrypi: new Nerves platform
Cache buildroot download to reduce rebuild time
Clean up .fw and .img file naming

This removes lots of hardcoded bbb.fw, etc. names and refactors the
post-createfs.sh code to nerves-common.
Merge pull request #4 from fhunleth-lkc/master

ag150: enable PM, multi-core, and RTC
Add RPi and x86 platforms to readme
post-createfs.sh: don't ignore errors

When fwtool fails, don't ignore the error and continue.
Remove rootfs.tar.gz creation to save time/space

For images with a lot of packages enabled, the tar.gz creation actually
takes a while. It isn't necessary since we can use the .tar file that's
created automatically anyway. This commit cleans that up.
Update unionfs download site