firmware-utils: kernel image generator for TP-Link RE450

The firmware image that is used in TP-Link RE450 (and some more devices from
the RE series) is tplink-safeloader.
In the kernel partition, the kernel is compressed in a regular tp-link
firmware that is just used for booting. Since it is only used for compressing
and booting, only four fields are filled in the header:
Vendor, version, kernel load address and kernel entry point.
mktplinkfw-kernel is a simpler version of mktpolinkfw that generate such
images. It also specifies the hardware id (as it is in the product info
section), so when doing a sysupgrade - the existing code will check for
hardware compatibility.

Signed-off-by: Tal Keren <kooolk@gmail.com>
[rd@radekdostal.com: add build target to .../image/tp-link.mk]
Signed-off-by: Radek Dostál <rd@radekdostal.com>