kirkwood: switch kernel version to 5.4

5.4 support is ready and tested.

Compile tested: all target devices
Run tested: pogoplug v4, nsa310b and two unofficial supported devices

Tested-by: Sungbo Eo <mans0n@gorani.run>
Tested-by: Alberto Bursi <alberto.bursi@outlook.it> [pogoplug v4 and nsa310b]
Signed-off-by: Pawel Dembicki <paweldembicki@gmail.com>
[fixed the switch, removed maintainer variable]
Signed-off-by: Petr Štetiar <ynezz@true.cz>

target/linux/kirkwood/Makefile

@@ -11,10 +11,8 @@ BOARD:=kirkwood
 BOARDNAME:=Marvell Kirkwood
 FEATURES:=usb nand squashfs ramdisk
 CPU_TYPE:=xscale
-MAINTAINER:=Luka Perkov <luka@openwrt.org>
 
-KERNEL_PATCHVER:=4.14
-KERNEL_TESTING_PATCHVER := 4.19
+KERNEL_PATCHVER:=5.4
 
 include $(INCLUDE_DIR)/target.mk