meta-agl-bsp: add "kingfisher" to AGL_FEATURES for -kf builds

meta-agl-bsp: add "kingfisher" to AGL_FEATURES for -kf builds

To be able to detect when building with Kingfisher support in
recipes, add a "kingfisher" feature to AGL_FEATURES.  This should be
lower impact on builds than an override wrt sharing sstate between
builds, and matches what has been done for the refhw support with
h3ulcb.

Bug-AGL: SPEC-4023

Signed-off-by: Scott Murray <scott.murray@konsulko.com>
Change-Id: I8f8eb148b94cdc8e80b0600b3112e45d1e709948
Reviewed-on: https://gerrit.automotivelinux.org/gerrit/c/AGL/meta-agl/+/26544
Tested-by: Jenkins Job builder account
ci-image-build: Jenkins Job builder account
ci-image-boot-test: Jenkins Job builder account
Reviewed-by: Jan-Simon Moeller <jsmoeller@linuxfoundation.org>