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>

meta/classes/base.bbclass

@@ -926,8 +926,8 @@ base_do_stage () {
 	:
 }
 
-do_populate_staging[dirs] = "${STAGING_DIR_TARGET}/${layout_bindir} ${STAGING_DIR_TARGET}/${layout_libdir} \
-			     ${STAGING_DIR_TARGET}/${layout_includedir} \
+do_populate_staging[dirs] = "${STAGING_DIR_TARGET}/${bindir} ${STAGING_DIR_TARGET}/${libdir} \
+			     ${STAGING_DIR_TARGET}/${includedir} \
 			     ${STAGING_BINDIR_NATIVE} ${STAGING_LIBDIR_NATIVE} \
 			     ${STAGING_INCDIR_NATIVE} \
 			     ${STAGING_DATADIR} \