ipq40xx: RT-AC58U: Try ARTIFACTS for install.trx

ipq40xx: RT-AC58U: Try ARTIFACTS for install.trx

Previous attempts of generating an "IMAGES" based approaches
to provide a ready-to-go image that can be flashed through
the vendor firmware's WEB-UI or via the bootloader Option 1
in order to perform a serial-console-less installation all
had the downsides. Either they rendered the INITRAMFS unusable
for the bootloader option 2, or broke the IMAGEBUILDER.

This hopefully does neither. The IMAGE_SIZE was changed to
account for the added 64 Byte U-Boot header.

WARNING: Hmm, this could/did break if the initramfs isn't available...
(But let's not forget it again that this is dangerous)

Signed-off-by: Christian Lamparter <chunkeey@gmail.com>