shill: rmtfs: Add shill user to rmtfs group

Allow shill to write into /sys/class/remoteproc/remoteproc*/state
which is owned by the rmtfs group.

BUG=b:168113309
TEST=Add shill to rmtfs group, flash new image and verify that shill
can reset the modem using the command:
gdbus call --system --dest=org.chromium.flimflam -o /device/rmnet_data0 -m org.chromium.flimflam.Device.Reset

Cq-Depend: chromium:2750583
Change-Id: I812fb756a036d7d14273c02ba6ad916a96075e59
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/overlays/eclass-overlay/+/2737683
Commit-Queue: Andrew Lassalle <andrewlassalle@chromium.org>
Tested-by: Andrew Lassalle <andrewlassalle@chromium.org>
Reviewed-by: Micah Morton <mortonm@chromium.org>
Reviewed-by: Allen Webb <allenwebb@google.com>
diff --git a/profiles/base/accounts/group/rmtfs b/profiles/base/accounts/group/rmtfs
index b57e594..a2adef7 100644
--- a/profiles/base/accounts/group/rmtfs
+++ b/profiles/base/accounts/group/rmtfs
@@ -1,3 +1,3 @@
 group:rmtfs
 gid:306
-users:rmtfs
+users:rmtfs,shill