ramips: mt7621: build .trx image for ASUS RT-AX53U

ramips: mt7621: build .trx image for ASUS RT-AX53U

Build an initramfs-factory.trx image suitable for flashing directly via
the vendor firmware WebGUI, without using TFTP or SSH.

To use:
1. Flash the initramfs-factory.trx file with the vendor firmware
   administrative interface.
2. Wait until the router restarts, login to OpenWRT via LuCI (or SSH)
   and upgrade firmware using the sysupgrade-squashfs.bin image file.

It is necessary to use TRX version 2, which has a kernel size limit of
16MB (TRX version 3 is rejected by vendor firmware). To avoid stopping
the whole build when the input file turns out larger, check in the
Build/asus-trx macro if check-size removed the input file and don't
error out in this case.

Note: It should be possible to prepare an image to flash the router in
one step, but this will be more complicated, because vendor firmware and
OpenWRT use a slightly different partition layout.

Signed-off-by: Mateusz Jończyk <mat.jonczyk@o2.pl>
Link: https://github.com/openwrt/openwrt/pull/23647
Signed-off-by: Robert Marko <robimarko@gmail.com>