mt7621: mikrotik: use vmlinuz (zBoot ELF)

- minimal built initramfs: 11MB vmlinux ELF -> 4.5MB vmlinuz
- ~5 seconds for kernel decompression, which was equivalent to the
  additional time to load the uncompressed ELF from SPI NOR.
- Removes requirement for lzma-loader, which may have been causing some
  image builds to fail to boot on Mikrotik mt7621.

Fixes: FS#3354
Suggested-by: Thibaut VARÈNE <hacks@slashdirt.org>
Signed-off-by: John Thomson <git@johnthomson.fastmail.com.au>

target/linux/ramips/image/mt7621.mk

@@ -708,8 +708,8 @@ define Device/MikroTik
   BLOCKSIZE := 64k
   IMAGE_SIZE := 16128k
   DEVICE_PACKAGES := kmod-usb3
-  LOADER_TYPE := elf
-  KERNEL := $(KERNEL_DTB) | loader-kernel
+  KERNEL_NAME := vmlinuz
+  KERNEL := kernel-bin | append-dtb-elf
   IMAGE/sysupgrade.bin := append-kernel | kernel2minor -s 1024 | \
 	pad-to $$$$(BLOCKSIZE) | append-rootfs | pad-rootfs | append-metadata | \
 	check-size

target/linux/ramips/mt7621/target.mk

@@ -6,6 +6,9 @@ SUBTARGET:=mt7621
 BOARDNAME:=MT7621 based boards
 FEATURES+=nand ramdisk rtc usb minor
 CPU_TYPE:=24kc
+KERNELNAME:=vmlinux vmlinuz
+# make Kernel/CopyImage use $LINUX_DIR/vmlinuz
+IMAGES_DIR:=../../..
 
 DEFAULT_PACKAGES += wpad-basic-wolfssl