bcm53xx: use iflag=skip_bytes for dd command during sysupgrade

Since BusyBox 1.25.0 dd command supports iflag=skip_bytes which allows
skipping requested amount of bytes without reducing blocksize. Thanks to
this we can leave default blocksize and let dd work more efficiently.

On Netgear R6250 "dd skip=58 iflag=skip_bytes" can be 5 times faster
than "dd bs=58 skip=1" when extracting TRX out of CHK.

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>