profiles: Disable split-usr for generic images completely

We do specify -split-usr in make.defaults for the USE variable but
it's not enough - the base profile in portage-stable forces this flag
and this overrides our defaults. As a workaround, we were using
package.use.force to force-disable split-usr for selected
packages. Now, with addition of more split-usr-using packages in
app-alternatives the list in package.use.force would grow. Instead of
listing all the packages having split-usr in their IUSE, use the
use.force and use.mask file to unforce and mask split-usr for all
packages in generic images.

As a bonus, this also allows us to drop a customization we did in
sys-apps/policycoreutils package.

sdk_container/src/third_party/coreos-overlay/profiles/coreos/targets/generic/package.use.force

@@ -1,4 +0,0 @@
-# We don't have a separate /{bin,lib} and /usr/{bin,lib}
-sys-apps/coreutils -split-usr
-sys-apps/systemd -split-usr
-sys-apps/shadow -split-usr

sdk_container/src/third_party/coreos-overlay/profiles/coreos/targets/generic/use.force

@@ -0,0 +1,4 @@
+# We don't have a separate /{bin,lib} and /usr/{bin,lib}. But the base
+# profile in portage-stable forces split-usr, so here we unforce it
+# and in use.mask we mask it.
+-split-usr

sdk_container/src/third_party/coreos-overlay/profiles/coreos/targets/generic/use.mask

@@ -0,0 +1,4 @@
+# We don't have a separate /{bin,lib} and /usr/{bin,lib}. But the base
+# profile in portage-stable forces split-usr, so in use.force we
+# unforce it and here we mask it.
+split-usr