ath79: image: fix initramfs for safeloader devices

Currently it's not possible to tftpboot initramfs image on archer-c7-v5
as the image contains tplink-v1-header which leads to:

 ath> bootm
 ## Booting image at 81000000 ...
 Bad Magic Number

as U-Boot expects uImage wrapped image. This is caused by following
inheritance issue:

  define Device/Init
    KERNEL_INITRAMFS = $$(KERNEL)

  define Device/tplink-v1
    KERNEL := kernel-bin | append-dtb | lzma
    KERNEL_INITRAMFS := kernel-bin | append-dtb | lzma | tplink-v1-header

  define Device/tplink-safeloader
    $(Device/tplink-v1)

  define Device/tplink-safeloader-uimage
    $(Device/tplink-safeloader)
    KERNEL := kernel-bin | append-dtb | lzma | uImageArcher lzma

  define Device/tplink_archer-c7-v5
    $(Device/tplink-safeloader-uimage)

where tplink-v1 defines KERNEL_INITRAMFS with tplink-v1-header and it's
then used by all devices inheriting from tplink-safeloader. Fix this by
overriding KERNEL_INITRAMFS to KERNEL variable again.

Signed-off-by: Petr Štetiar <ynezz@true.cz>
(cherry picked from commit ceeece9ffaa5a3a336505332c39794d76c08b2ca)

target/linux/ath79/image/common-tp-link.mk

@@ -102,6 +102,7 @@ endef
 define Device/tplink-safeloader
   $(Device/tplink)
   KERNEL := kernel-bin | append-dtb | lzma | tplink-v1-header -O
+  KERNEL_INITRAMFS := $$(KERNEL)
   IMAGE/sysupgrade.bin := append-rootfs | tplink-safeloader sysupgrade | \
     append-metadata | check-size $$$$(IMAGE_SIZE)
   IMAGE/factory.bin := append-rootfs | tplink-safeloader factory
@@ -110,4 +111,5 @@ endef
 define Device/tplink-safeloader-uimage
   $(Device/tplink-safeloader)
   KERNEL := kernel-bin | append-dtb | lzma | uImageArcher lzma
+  KERNEL_INITRAMFS := $$(KERNEL)
 endef