If SDKMACHINE is set then check that a configuration file matching it
actually exists, otherwise the user won't know that they've set it
incorrectly.
(From OE-Core rev: 8c984f92af821a4048c93f8e308c5f4a3fa39ca4)
Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>