lantiq: ar7: use okli loader for FRITZ!Box

With Kernel 5.10 the ar7 FRITZ!Box are not booting the initramfs nor the
sysupgrade image any more. Presumably due to the grown kernel.

Use the okli preloader to workaround the bootloader issue. No solution
so far for the initramfs.

Signed-off-by: Mathias Kresin <dev@kresin.me>

target/linux/lantiq/image/Makefile

@@ -49,12 +49,35 @@ define Build/fullimage
 	rm $@.tmp
 endef
 
-DEVICE_VARS += SIGNATURE MAGIC CRC32_POLY
+define Build/loader-okli-compile
+	rm -rf $@.src
+	$(MAKE) -C lzma-loader \
+		PKG_BUILD_DIR="$@.src" \
+		TARGET_DIR="$(dir $@)" LOADER_NAME="$(notdir $@)" \
+		PLATFORM="lantiq" \
+		LZMA_TEXT_START=0x82000000 \
+		LOADADDR=0x80002000 \
+		FLASH_START=0x10000000 \
+		FLASH_OFFS=$(LOADER_FLASH_OFFS) \
+		FLASH_MAX=0x0 \
+		$(1) compile loader.bin
+	mv "$@.bin" "$@"
+	rm -rf $@.src
+endef
+
+define Build/prepend-loader-okli
+	cat "$(KDIR)/loader-$(word 1,$(1)).bin" "$@" >> "$@.new"
+	mv "$@.new" "$@"
+endef
+
+DEVICE_VARS += SIGNATURE MAGIC CRC32_POLY LOADER_FLASH_OFFS
+
 DTS_DIR := $(DTS_DIR)/lantiq
 
 # Shared device definition: applies to every defined device
 define Device/Default
   PROFILES = Default
+  COMPILE :=
   KERNEL_DEPENDS = $$(wildcard $(DTS_DIR)/$$(DEVICE_DTS).dts)
   KERNEL_INITRAMFS_NAME = $$(KERNEL_NAME)-initramfs
   KERNEL := kernel-bin | append-dtb | lzma | uImage lzma
@@ -105,6 +128,16 @@ define Device/AVM
 	append-rootfs | pad-rootfs | check-size | append-metadata
 endef
 
+define Device/AVM_preloader
+  DEVICE_VENDOR := AVM
+  COMPILE := loader-$(1).bin
+  COMPILE/loader-$(1).bin := loader-okli-compile | lzma | eva-image | \
+	pad-to 64k | append-avm-fakeroot | pad-to 4k
+  KERNEL := kernel-bin | append-dtb | lzma | uImage lzma -M 0x4f4b4c49 | \
+	prepend-loader-okli $(1)
+  KERNEL_INITRAMFS := kernel-bin | append-dtb | lzma | eva-image
+endef
+
 ifeq ($(SUBTARGET),ase)
 DEFAULT_SOC := amazonse
 include amazonse.mk