bcm63xx: use kernel 5.4 as testing kernel

This uses 5.4 as testing kernel. Since 4.19 has not seen broad
testing yet, just keep 4.14 as stable kernel until 5.4 is ready.

Tested on Comtrend AR-5387un (Thanks to @Noltari).

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

target/linux/bcm63xx/Makefile

@@ -13,7 +13,7 @@ BOARDNAME:=Broadcom BCM63xx
 SUBTARGETS:=generic smp
 FEATURES:=squashfs usb atm pci pcmcia usbgadget
 KERNEL_PATCHVER:=4.14
-KERNEL_TESTING_PATCHVER:=4.19
+KERNEL_TESTING_PATCHVER:=5.4
 
 define Target/Description
 	Build firmware images for Broadcom based xDSL/routers