Remove layout_* variables

Remove layout_* variables

Remove layout_* variables and replace them with variables specific to the
different classes. The layout variables were only useful for the native/cross
classes and caused more confusion than they solved. They didn't scale to the
sdk class. It now clear a small set of native/cross variables fulfil the needs.

This patch also changes native.bbclass to use "/" as the STAGING_DIR which makes
sense since we're installing binaries into the locations we're compiling them for.

Signed-off-by: Richard Purdie <rpurdie@linux.intel.com>