bitbake: toaster: remove SDKMACHINE from project variables

bitbake: toaster: remove SDKMACHINE from project variables

Removed SDKMACHINE from predefined set of variables
for the project as it causes bitbake build error:
   SDKMACHINE is set, but SDK_ARCH has not been changed as a result

This variable does not need to be predefined as it's not used by
toaster.

It's still possible to specify it in project configuration
if needed. SDK_ARCH variable should be set too to avoid above
mentioned build failure.

(Bitbake rev: a6ab2a38ab666376b4a5e796d3a88929e8cbfb22)

Signed-off-by: Ed Bartosh <ed.bartosh@linux.intel.com>
Signed-off-by: brian avery <avery.brian@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>