base-files, metadata: support additional group membership

Some packages may require additional group membership for the system
user added by that package. Allow defining additional groups as third
member of the ':'-separated tuple, allowing to specify multiple
','-separated groups with optional GID.

Example:
USERID:=foouser=1000:foogroup=1000:addg1=1001,addg2=1002,addg3

Signed-off-by: Daniel Golle <daniel@makrotopia.org>

package/base-files/files/lib/functions.sh

@@ -209,10 +209,10 @@ add_group_and_user() {
 	if [ -n "$rusers" ]; then
 		local tuple oIFS="$IFS"
 		for tuple in $rusers; do
-			local uid gid uname gname
+			local uid gid uname gname addngroups addngroup addngname addngid
 
 			IFS=":"
-			set -- $tuple; uname="$1"; gname="$2"
+			set -- $tuple; uname="$1"; gname="$2"; addngroups="$3"
 			IFS="="
 			set -- $uname; uname="$1"; uid="$2"
 			set -- $gname; gname="$1"; gid="$2"
@@ -232,7 +232,24 @@ add_group_and_user() {
 				group_add_user "$gname" "$uname"
 			fi
 
-			unset uid gid uname gname
+			if [ -n "$uname" ] &&  [ -n "$addngroups" ]; then
+				oIFS="$IFS"
+				IFS=","
+				for addngroup in $addngroups ; do
+					IFS="="
+					set -- $addngroup; addngname="$1"; addngid="$2"
+					if [ -n "$addngid" ]; then
+						group_exists "$addngname" || group_add "$addngname" "$addngid"
+					else
+						group_add_next "$addngname"
+					fi
+
+					group_add_user "$addngname" "$uname"
+				done
+				IFS="$oIFS"
+			fi
+
+			unset uid gid uname gname addngroups addngroup addngname addngid
 		done
 	fi
 }