firmware-utils: tplink-safeloader: add dynamic partitions

firmware-utils: tplink-safeloader: add dynamic partitions

Add support to dynamically split the firmware partition into os-image
and file-system partitions. This is done by replacing those entries in
the partition table with a single unified firmware partition, which is
then split according to actual kernel image size.

The factory image will have the file-system partition aligned to a 64K
erase block, but the sysupgrade image skips this and aligns only the
JFFS2 EOF marker to squeeze out more space.

This should prevent further creeping updates to the kernel partition
size while maximizing space for the overlay filesystem on smaller
devices.

Signed-off-by: Alex Maclean <monkeh@monkeh.net>
(backported from 408c54b02f6a6dd2c0ae30a7885a37c9ac4c1b53)