cmake: extensions: add dt_alias()

This is a helper function for looking up the path to a devicetree
alias. It is analogous to dt_chosen(). It has to exist as a separate
function from dt_prop() for similar reasons that dt_chosen() does:
the edtlib APIs for interacting with /aliases are special-cased in the
same way they are for /chosen.

Signed-off-by: Martí Bolívar <marti.bolivar@nordicsemi.no>