opkg: fix stray \ warnings with grep-3.8

We simply grep for "src/". So no need for "\/".
Furthermore, since grep-3.8 this creates warnings.

As written in the grep-3.8 announcement:
  Regular expressions with stray backslashes now cause warnings, as
  their unspecified behavior can lead to unexpected results.
  For example, '\a' and 'a' are not always equivalent
  <https://bugs.gnu.org/39678>.

Fixes a warning during the first boot:
  grep: warning: stray \ before /

Signed-off-by: Georgi Valkov <[email protected]>

package/system/opkg/files/20_migrate-feeds

@@ -1,6 +1,6 @@
 #!/bin/sh
 
-[ -f /etc/opkg.conf ] && grep -q "src\/" /etc/opkg.conf || exit 0
+[ -f /etc/opkg.conf ] && grep -q "src/" /etc/opkg.conf || exit 0
 
 echo -e "# Old feeds from previous image\n# Uncomment to reenable\n" >> /etc/opkg/customfeeds.conf
 sed -n "s/.*\(src\/.*\)/# \1/p" /etc/opkg.conf >> /etc/opkg/customfeeds.conf