sdk: assert host system is running Linux 3.7 or later

Currently building images on older kernels will fail because mkfs.btrfs
enables an incompatible feature 'extref' by default. We never really
made this requirement explicit and the SDK in general has continued to
maintain compatibility with older kernels. Make the requirement explicit
so users will get errors quicker and there is a clear line for what
kernel features can be used in the SDK.
Merge pull request #326 from marineam/kernel

sdk: assert host system is running Linux 3.7 or later