realtek: fix ZyXEL initramfs image generation

realtek: fix ZyXEL initramfs image generation

The current rule produces empty trailers, causing the OEM firmware
update application to reject our images.

The double expansion of a makefile variable does not work inside
shell code.  The second round is interpreted as a shell expansion,
attempting to run the command ZYXEL_VERS instead of expanding the
$(ZYXEL_VERS) makefile variable.

Fix by removing one level of variable indirection.

Fixes: c6c8d597e183 ("realtek: Add generic zyxel_gs1900 image definition")
Tested-by: Sander Vanheule <sander@svanheule.net>
Signed-off-by: Bjørn Mork <bjorn@mork.no>