ipq40xx: add testing support for kernel 5.10

ipq40xx: add testing support for kernel 5.10

Add kernel 5.10 as the testing kernel to ipq40xx to
get wider testing.

The following devices failed to build with buildbot settings and all
feeds installed (apparently due to kernel size):

 * cell-c rtl30vw
 * compex wpj428
 * devolo magic 2 next
 * engenius emr3500
 * glinet gl-b1300
 * glinet gl-s1300
 * qcom ap-dk01.1-c1
 * qcom ap-dk04.1-c1

Signed-off-by: Robert Marko <robert.marko@sartura.hr>
Tested-by: Nick Hainke <vincent@systemli.org> [ipq4019/fritzbox-7530
                                               ipq4019/fritzbox-4040
                                               ipq4019/sxtsq-5ac]
Tested-by: Szabolcs Hubai <szab.hu@gmail.com> [ipq4029/gl-b1300]
Tested-by: Stefan Lippers-Hollmann <s.l-h@gmx.de> [ipq4019/map-ac2200]
[add tested-by and note about failed devices]
Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>