Makefile: use /etc/os-release for version info rather than /etc/br-version

Makefile: use /etc/os-release for version info rather than /etc/br-version

/etc/os-release is becoming a standard interface for distribution
name/version info, so let's use that instead of the nonstandard
/etc/br-version. Format of the file is something like:

NAME=Buildroot
VERSION=2012.02-rc1-00003-g2d10e81
ID=buildroot
VERSION_ID=2012.02-rc1
PRETTY_NAME="Buildroot 2012.02-rc1"

For more details, see:

http://0pointer.de/public/systemd-man/os-release.html

Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>