realtek: imagebuilder: add guard around loader generation

realtek: imagebuilder: add guard around loader generation

The imagebuilder cannot compile source, so we must guard against
generation of artifact targets that require this.  Without this
guard we see an error when building the zyxel,gs1920-24hp-v1 profile.

  Create standalone rt-loader, loading uimage from address 0xb40c0000
  mips-openwrt-linux-musl-gcc -fpic -msoft-float -Iinclude -c -o ... src/startup.S
  make[4]: mips-openwrt-linux-musl-gcc: No such file or directory

Signed-off-by: Eric Fahlgren <ericfahlgren@gmail.com>
Link: https://github.com/openwrt/openwrt/pull/21556
Signed-off-by: Robert Marko <robimarko@gmail.com>