realtek: switch Zyxel GS1900 initramfs recipe to rt-loader

These devices need a tiny (<8MB) initramfs. There are first
occurrences where this fails with newer kernels and diagnostic
packages.

Switch the recipe over to use lzma compression and rt-loader.

Signed-off-by: Markus Stockhausen <[email protected]>
Link: https://github.com/openwrt/openwrt/pull/19687
Signed-off-by: Hauke Mehrtens <[email protected]>

target/linux/realtek/image/common.mk

@@ -86,8 +86,9 @@ define Device/zyxel_gs1900
   KERNEL_INITRAMFS := \
 	kernel-bin | \
 	append-dtb | \
-	libdeflate-gzip | \
+	rt-compress | \
 	zyxel-vers | \
-	uImage gzip | \
+	rt-loader | \
+	uImage none | \
 	check-size 6976k
 endef