onie-support:  allow for arch and platform specific support contributions

onie-support now optionally includes architecture and platform
specific details in the support tarball.

The arch and platform specific shell script fragments are loaded from
/lib/onie/support-arch and /lib/onie/support-arch respectively.

To include arch specific information implement the support_arch()
function.  Similarly for platform specific information implement the
support_platform() function.