yellow: Only install stable packages on the host

Since the container images we're using as a base for the build system
only contain stable packages, setting ACCEPT_KEYWORDS to allow unstable
packages globally can cause a lot of rebuilds and potentially break
things.  Instead, we only set ~arch for the packages we actually need
recent versions on the host.

This does not affect packages installed in the target root, of course.

yellow/portage/host/etc/portage/make.conf/15-keywords.conf

@@ -1 +1 @@
-ACCEPT_KEYWORDS="~${ARCH} ${ARCH}"
+ACCEPT_KEYWORDS="${ARCH}"

yellow/portage/host/etc/portage/package.accept_keywords/genimage

@@ -0,0 +1 @@
+sys-fs/genimage ~amd64