arm64: boot: add zstd support

Support build the zstd compressed Image.zst. Similar as other
compressed formats, the Image.zst is not self-decompressing and
the bootloader still needs to handle decompression before
launching the kernel image.
Signed-off-by: Jisheng Zhang <jszhang@kernel.org>
Link: https://lore.kernel.org/r/20220619170657.2657-1-jszhang@kernel.orgSigned-off-by: Will Deacon <will@kernel.org>

arch/arm64/boot/Makefile

@@ -16,7 +16,7 @@
 
 OBJCOPYFLAGS_Image :=-O binary -R .note -R .note.gnu.build-id -R .comment -S
 
-targets := Image Image.bz2 Image.gz Image.lz4 Image.lzma Image.lzo
+targets := Image Image.bz2 Image.gz Image.lz4 Image.lzma Image.lzo Image.zst
 
 $(obj)/Image: vmlinux FORCE
 	$(call if_changed,objcopy)
@@ -35,3 +35,6 @@ $(obj)/Image.lzma: $(obj)/Image FORCE
 
 $(obj)/Image.lzo: $(obj)/Image FORCE
 	$(call if_changed,lzo)
+
+$(obj)/Image.zst: $(obj)/Image FORCE
+	$(call if_changed,zstd)