eclass-overlay: add user and group for image-burner

This is the first step to sandbox image-burner process.

Take a UID/GID that ranges from 300-399 from ./display-accts.py --show-free as I assume image-burner needs to talk to filesystems.

BUG=chromium:766130
TEST=Build image with updated chromeos-base/chromeos-imageburner, check
image-burner is in /etc/passwd and /etc/group.

Change-Id: I05cdf5f0385c7757652a64bbecceb85cb7a4e408
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/overlays/eclass-overlay/+/2823058
Reviewed-by: Allen Webb <allenwebb@google.com>
Tested-by: Zi Lin <lziest@chromium.org>
Commit-Queue: Allen Webb <allenwebb@google.com>
diff --git a/profiles/base/accounts/group/image-burner b/profiles/base/accounts/group/image-burner
new file mode 100644
index 0000000..eb12954
--- /dev/null
+++ b/profiles/base/accounts/group/image-burner
@@ -0,0 +1,3 @@
+group:image-burner
+gid:310
+users:image-burner
diff --git a/profiles/base/accounts/user/image-burner b/profiles/base/accounts/user/image-burner
new file mode 100644
index 0000000..b4936d5
--- /dev/null
+++ b/profiles/base/accounts/user/image-burner
@@ -0,0 +1,6 @@
+user:image-burner
+uid:310
+gid:310
+gecos:CrOS Image Burner daemon
+home:/dev/null
+shell:/bin/false