efi: generalize (stage2 += rootfs)

This feature is more generally applicable indeed;
might result in duplication due to the installer
components adding "efivars" independently but that
is to be sorted out later in those components:
- check whether it's added already sometime soon;
- maybe stop adding that at some point in the future.

install2 and rescue roots still need this too though.

features.in/efi/rootfs/image-scripts.d/50-efi

@@ -0,0 +1,4 @@
+#!/bin/sh
+# see also https://bugzilla.altlinux.org/27970
+
+grep -q efivars /etc/modules || echo efivars >> /etc/modules

features.in/efi/stage2/image-scripts.d/50-efi

@@ -1,4 +1,4 @@
 #!/bin/sh
 # see also https://bugzilla.altlinux.org/27970
 
-echo efivars >> /etc/modules
+grep -q efivars /etc/modules || echo efivars >> /etc/modules