i586: Only add the current tune to PACKAGE_EXTRA_ARCHS

i586: Only add the current tune to PACKAGE_EXTRA_ARCHS

The generic x86 build supports i586 by default, so this specific tune
file technically doesn't add any specific ARCHes to PACKAGE_EXTRA_ARCHS.
For consistency, append the current tune to PACKAGE_EXTRA_ARCHS.

Since we do not have specific tune files for i386 and i486, just drop
them.

These could be added to tune-x86 version if there is a need to
maintain them, but they really do not belong here.

(From OE-Core rev: 1ff914118bdfb19d7f3d794a92ba3735c06ab97b)

Signed-off-by: Darren Hart <dvhart@linux.intel.com>
Cc: Richard Purdie <richard.purdie@linuxfoundation.org>
Cc: Paul Eggleton <paul.eggleton@intel.com>
Cc: Tom Zanussi <tom.zanussi@intel.com>
Cc: Nitin Kamble <nitin.a.kamble@intel.com>
Cc: Mark Hatle <mark.hatle@windriver.com>
Cc: Bruce Ashfield <bruce.ashfield@windriver.com>
Cc: Martin Jansa <martin.jansa@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>