ath79: allow skipping hash for Senao sysupgrade

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>