ipq806x: unset filesystem specific build command by default

Initialise the filesystem specific build command. Otherwise it will be
unintended inherit to following images and will cause a wrong image
format.

The issue is around for ages but was only recently triggered due to the
added support for the Openmesh a42.

Fixes: FS#1276

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

target/linux/ipq806x/image/Makefile

@@ -31,6 +31,7 @@ define Device/Default
 	KERNEL_LOADADDR = 0x42208000
 	SUPPORTED_DEVICES := $(subst _,$(comma),$(1))
 	IMAGE/sysupgrade.bin = sysupgrade-tar | append-metadata
+	IMAGE/sysupgrade.bin/squashfs :=
 endef
 
 define Device/LegacyImage