build: add a script for generating Linksys factory images

build: add a script for generating Linksys factory images

This commit adds the 'Build/linksys-image' rule and the
'linksys-image.sh' script to the build system.

This change is needed for generating factory images for the Linksys
EA6350v3 device. Without this patch, only valid sysupgrade images can be
generated. With this patch, users can flash the device without the
need of physical access or disassembly.

Signed-off-by: Ryan Pannell <ryan@osukl.com>
Signed-off-by: Oever González <notengobattery@gmail.com>