base.bbclass: Add PKGTRIPLETS and PKGMLTRIPLETS variables
These variables correspond to the PACKAGE_ARCH list combined with the TARGET_VENDOR
and TARGET_OS values. These can be used to traverse the pkgdata structure.
Setting these once in base.bbclass stops pkgdata needing to recalculate the values
and is also useful for the reworked shlibs code in a patch that will follow this.
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
