ath79: allow skipping hash for Senao sysupgrade

Some vendors of Senao boards have a similar flash layout
situation that causes the need to split the firmware partition
and use the lzma-loader, but do not store
checksums of the partitions or otherwise
do not even have a uboot environment partition.

This adds simple shell logic to skip that part.

Also, simplify some lines and variable usage.

Signed-off-by: Michael Pratt <mcpratt@pm.me>

target/linux/ath79/generic/base-files/lib/upgrade/platform.sh

@@ -45,6 +45,7 @@ platform_do_upgrade() {
 	watchguard,ap100|\
 	watchguard,ap200|\
 	watchguard,ap300)
+		ENV_SCRIPT="/tmp/fw_env"
 		IMAGE_LIST="tar tzf $1"
 		IMAGE_CMD="tar xzOf $1"
 		KERNEL_PART="loader"