umbim: add "_proto_mbim_get_field" helper

Add a new helper to extract IP configuration from umbim output. This is
required to extract fields which can possibly have multiple values,
namely IP addresses and DNS servers, and get rid of primitive parser
using 'eval' builtin without support for this.

Signed-off-by: Lech Perczak <lech.perczak@gmail.com>

package/network/utils/umbim/files/lib/netifd/proto/mbim.sh

@@ -26,6 +26,18 @@ proto_mbim_init_config() {
 	proto_config_add_defaults
 }
 
+_proto_mbim_get_field() {
+        local field="$1"
+        shift
+        local mbimconfig="$@"
+        echo "$mbimconfig" | while read -r line; do
+                variable=${line%%:*}
+                [ "$variable" = "$field" ] || continue;
+                value=${line##* }
+                echo -n "$value "
+        done
+}
+
 _proto_mbim_setup() {
 	local interface="$1"
 	local tid=2