realtek: Migrate to libdeflate

Libdeflate is a more advanced gzip compressor, which allows for faster
decompression, higher compression speed (factor 3-4), while being fully
gzip compatible.

Some comparison
gzip    | libdeflate-gzip | delta  | image [openwrt-realtek-rtl839x-*]
--------+-----------------+--------+-----------------------------------------------
6589174 | 6298794         | 290380 | d-link_dgs-1210-52-initramfs-kernel.bin
6291632 | 6029488         | 262144 | d-link_dgs-1210-52-squashfs-factory_image1.bin
6292270 | 6030128         | 262142 | d-link_dgs-1210-52-squashfs-sysupgrade.bin
6589142 | 6298760         | 290382 | zyxel_gs1900-48-initramfs-kernel.bin
6292264 | 6030122         | 262142 | zyxel_gs1900-48-squashfs-sysupgrade.bin

and changing lzma to (libdeflate-)gzip on existing rtl930x target:
gzip    | libdeflate-gzip | delta  | image [openwrt-realtek-rtl930x-*]
--------+-----------------+--------+--------------------------------------
6816230 | 6510382         | 305848 | zyxel_xgs1250-12-initramfs-kernel.bin

Signed-off-by: Olliver Schinagl <[email protected]>
Reviewed-by: Robert Marko <[email protected]>
Reviewed-by: Rosen Penev <[email protected]>
Reviewed-by: Sander Vanheule <[email protected]>

target/linux/realtek/image/Makefile

@@ -90,12 +90,12 @@ define Device/Default
   KERNEL := \
 	kernel-bin | \
 	append-dtb | \
-	gzip | \
+	libdeflate-gzip | \
 	uImage gzip
   KERNEL_INITRAMFS := \
 	kernel-bin | \
 	append-dtb | \
-	gzip | \
+	libdeflate-gzip | \
 	uImage gzip
   DEVICE_DTS_DIR := ../dts-$(KERNEL_PATCHVER)
   DEVICE_DTS = $$(SOC)_$(1)

target/linux/realtek/image/common.mk

@@ -7,7 +7,7 @@ define Device/d-link_dgs-1210
   KERNEL := \
 	kernel-bin | \
 	append-dtb | \
-	gzip | \
+	libdeflate-gzip | \
 	uImage gzip | \
 	dlink-cameo
   CAMEO_KERNEL_PART := 2
@@ -60,7 +60,7 @@ define Device/zyxel_gs1900
   KERNEL_INITRAMFS := \
 	kernel-bin | \
 	append-dtb | \
-	gzip | \
+	libdeflate-gzip | \
 	zyxel-vers | \
 	uImage gzip
 endef

target/linux/realtek/image/rtl838x.mk

@@ -70,7 +70,7 @@ define Device/engenius_ews2910p
   KERNEL_INITRAMFS := \
 	kernel-bin | \
 	append-dtb | \
-	gzip | \
+	libdeflate-gzip | \
 	uImage gzip -n 'IMG-0.00.00-c0.0.00'
 endef
 TARGET_DEVICES += engenius_ews2910p