conf/machine/tune: Overhaul tune include file variables

conf/machine/tune: Overhaul tune include file variables

There is currently consideradble confusion over how the tune files operate
and how these interact with the rest of the build system. This update/overhaul
changes things so the tune files are primarily resonsible for setting:

TUNE_ARCH - What was formerly set as TARGET_ARCH and is the value that
represents the architecture we're targetting.

TUNE_PKGARCH - The value that represents the tune confuration that this set
of tune parameters results in.

This allows the significant improvement that the core can now always determine
the target architecture value, even when TARGET_ARCH needs to be reset to
something different and likewise, there is one package architecture variable
the core can reference allowing simplification of the BASE_PACKAGE_ARCH, PACKAGE_ARCH
and FEED_ARCH variables.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>