pxe: add support for per arch and SoC default paths

A pxelinux server setup for "default" menu is typically an x86 binary.
This does not work well with a mixed architecture setup. Extend the default
search to look for default-<arch>-<soc> and then default-<arch> before
falling back to just "default".

Signed-off-by: Rob Herring <rob.herring@calxeda.com>