firmware-utils: mktplinkfw: fix JFFS2 EOF markers

firmware-utils: mktplinkfw: fix JFFS2 EOF markers

mktplinkfw/mktplinkfw2 utilities put JFFS2 EOF market only at 64KB
boundary, this could lead to current device configuration lost during
the sysupgrade on a device, which is equpped with flash with the 4KB
erase block size (e.g. TP-Link Archer C20).

This happens when 64KB and 4KB alignments do not match, so the JFFS2
data is written not exactly at the partition beginnig and startup
scripts can not find the JFFS2 during the first boot just after the
sysupgrade.

Fix this by placing additional JFFS2 EOF marker at a 4KB boundary. Also
keep the marker at 64KB intact, so the utilities will produce images
suitable for devices with both 4KB and 64KB erase blocks.

Fixes: 29a2c2ea80441895a2ffe100d854d2b26d5fa606 (add ability to put
jffs2 eof marker into the image)

Signed-off-by: Sergey Ryazanov <ryazanov.s.a@gmail.com>