ath79: optimize the firmware recipe for Netgear NAND devices

ath79: optimize the firmware recipe for Netgear NAND devices

1. Drop useless character '0xff' before fake filesystem header.
2. Reduce useless padding to shrink the size of the sysupgrade image.
3. Do not check the size of sysupgrade image. It does not make sense to
   check the size of a compressed package.
4. Do not take the size of netgear header into account because it will
   not be written to Flash.
5. Use the default lzma compression dictionary parameter '-d24' to get
   better performance.

Tested on Netgear R6100
Signed-off-by: Shiji Yang <yangshiji66@qq.com>