busybox: Only apply the patch on busybox 1.21

The ifplugd patch was backported and doesn't need to be applied
on later versions.

recipes-core/busybox/busybox_sysmocom_1.21.1.inc

@@ -0,0 +1,2 @@
+SRC_URI += "file://ifplugd-use-a-larger-netlink-buffer.patch"
+PRINC := "${@int(PRINC) + 1}"

recipes-core/busybox/busybox_sysmocom_systemd.inc

@@ -2,5 +2,3 @@
 SYSMOCOM_D := "${@os.path.dirname(bb.data.getVar('FILE', d, True))}"
 FILESEXTRAPATHS_prepend := "${SYSMOCOM}/${PN}-systemd:${SYSMOCOM_D}/${PN}:"
 PRINC := "${@int(PRINC) + 3}"
-
-SRC_URI += "file://ifplugd-use-a-larger-netlink-buffer.patch"

yocto-dora/busybox_1.21.1.bbappend

@@ -2,3 +2,4 @@
 SYSMOCOM_ORIG_PV := "${PV}"
 require recipes-core/busybox/${PN}_sysmocom.inc
 require recipes-core/busybox/${PN}_sysmocom_systemd.inc
+require recipes-core/busybox/${PN}_sysmocom_1.21.1.inc