Firstly we should start checking if MACHINE is set. It if isn't lets
make this a sanity warning since its very hard to successfully build
without setting one (and anyone wanting to do that can disable the
sanity checks easily enough anyway).
Some of the checks depend on a MACHINE being specified. This change
moves those checks to a separate function so they only run if MACHINE
has been set correctly.
Both these issues combine to ensure the user sees a sane message
and avoids the nasty tracebacks in the bug report referenced above.
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
