meta-agl-bsp: Enable building agl-refhw-h3 firmware

meta-agl-bsp: Enable building agl-refhw-h3 firmware

Changes:
- Replace the agl-refhw-h3 addition to MACHINEOVERRIDES with just
  using AGL_FEATURES.  With the rework of the arm-trusted-firmware
  bbappend in meta-agl-refhw, the override is no longer necessary,
  and a feature in AGL_FEATURES makes more sense in the context of
  this not being a separate machine in the AGL build, but just a
  feature.
- Remove BBMASK so the arm-trusted-firmware bbappend from
  meta-agl-refhw-gen3 will now be picked up.
- Hook up building the U-Boot variant for the refhw by extending
  UBOOT_CONFIG.
- Remove the compatible machine declaration from gpsd-conf-refhw,
  as we're using the package in h3ulcb images anyways.

Bug-AGL: SPEC-3916

Signed-off-by: Scott Murray <scott.murray@konsulko.com>
Change-Id: Ibe3ecdbe74811793c0e030df28a64b3052d64f47
Reviewed-on: https://gerrit.automotivelinux.org/gerrit/c/AGL/meta-agl/+/26476
Reviewed-by: Jan-Simon Moeller <jsmoeller@linuxfoundation.org>
Tested-by: Jan-Simon Moeller <jsmoeller@linuxfoundation.org>