ipq806x: switch to 4.19 kernel version

It has been used by several people for some time already and feedback
has been mostly positive.

Ref: https://github.com/openwrt/openwrt/pull/2472
Tested-by: Hannu Nyman <hannu.nyman@iki.fi> [ipq8065, R7800]
Tested-by: Stefan Lippers-Hollmann <s.l-h@gmx.de> [ipq8065, NBG6817]
Signed-off-by: Ansuel Smith <ansuelsmth@gmail.com>
[separate commit, commit subject and description facelift]
Signed-off-by: Petr Štetiar <ynezz@true.cz>

target/linux/ipq806x/Makefile

@@ -11,7 +11,7 @@ CPU_SUBTYPE:=neon-vfpv4
 MAINTAINER:=John Crispin <john@phrozen.org>
 SUBTARGETS:=generic
 
-KERNEL_PATCHVER:=4.14
+KERNEL_PATCHVER:=4.19
 
 KERNELNAME:=zImage Image dtbs