sdk/DefaultBoard: Use portage arch for default
sdk/DefaultBoard: Use portage arch for default

In preparation for running kola on other architectures.

Signed-off-by: Geoff Levand <c3918f39022407dee6a0056ceca9b1aa80e8e4ce@infradead.org>

dm0-/mantle,glevand/coreos--mantle,dm0-/mantle,glevand/coreos--mantle