ramips: add linux zboot support

linux-mips has zboot code which can create a self-extracting kernel
image.
This allows enabling kernel zboot support for ramips targets.

Signed-off-by: Chuanhong Guo <gch981213@gmail.com>

target/linux/ramips/patches-5.4/0122-mips-ralink-enable-zboot-support.patch

@@ -0,0 +1,26 @@
+From 1f0400d0e2c410b04f246aefb2e9b5155eb4b0bf Mon Sep 17 00:00:00 2001
+From: Chuanhong Guo <gch981213@gmail.com>
+Date: Tue, 13 Oct 2020 10:05:47 +0800
+Subject: mips: ralink: enable zboot support
+
+Some of these ralink devices come with an ancient u-boot which can't
+extract LZMA properly when image gets too big.
+Enable zboot support to get a self-extracting kernel instead of relying
+on broken u-boot support.
+
+Signed-off-by: Chuanhong Guo <gch981213@gmail.com>
+Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>
+---
+ arch/mips/Kconfig | 1 +
+ 1 file changed, 1 insertion(+)
+
+--- a/arch/mips/Kconfig
++++ b/arch/mips/Kconfig
+@@ -625,6 +625,7 @@ config RALINK
+ 	select SYS_SUPPORTS_32BIT_KERNEL
+ 	select SYS_SUPPORTS_LITTLE_ENDIAN
+ 	select SYS_SUPPORTS_MIPS16
++	select SYS_SUPPORTS_ZBOOT
+ 	select SYS_HAS_EARLY_PRINTK
+ 	select CLKDEV_LOOKUP
+ 	select ARCH_HAS_RESET_CONTROLLER