arm-trusted-firmware: build 1rank firmware when MACHINE=agl-refhw-h3

Tweak the arm-trusted-firmware bbappend to build the extra 1rank
firmware when building standalone with MACHINE=agl-refhw-h3, and
also add an anonymous python snippet to disable the default
firmware configurations from the base recipe in that situation
as well, as they are not useful.

Bug-AGL: SPEC-4115

Signed-off-by: Scott Murray <scott.murray@konsulko.com>
Change-Id: I4697476b08c9dfc722cc8e97c837786874869262