base-files: replace backticks by $(...)

This replaces deprecated backticks by more versatile $(...) syntax.

This does not touch lib/upgrade/nand.sh, as there replacement is
not trivial.

Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>

package/base-files/files/etc/uci-defaults/13_fix-group-user

@@ -1,6 +1,6 @@
 . /lib/functions.sh
 
-for file in `grep -sl Require-User /usr/lib/opkg/info/*.control`; do
+for file in $(grep -sl Require-User /usr/lib/opkg/info/*.control); do
 	file="${file##*/}"
 	file="${file%.control}"
 	add_group_and_user "${file}"