ath79: meraki-mr18: fix initramfs build

Now, that initramfs images built for ZTE devices work, by moving
LZMA_TEXT_START further up the available RAM - same fix works
successfully for Meraki MR18 too. Apply it and reenable initramfs
generation again.

Fixes: 1d49310 ("ath79: add Cisco Meraki MR18")
Signed-off-by: Lech Perczak <lech.perczak@gmail.com>
Link: openwrt#17680
Signed-off-by: Robert Marko <robimarko@gmail.com>

target/linux/ath79/image/nand.mk

@@ -313,10 +313,9 @@ define Device/meraki_mr18
   BLOCKSIZE := 128k
   PAGESIZE := 2048
   LOADER_TYPE := bin
+  LZMA_TEXT_START := 0x82800000
   KERNEL := kernel-bin | append-dtb | lzma | loader-kernel | meraki-header MR18
-# Initramfs-build fails due to size issues
-# KERNEL_INITRAMFS := $$(KERNEL)
-  KERNEL_INITRAMFS :=
+  KERNEL_INITRAMFS := $$(KERNEL)
   IMAGE/sysupgrade.bin := sysupgrade-tar | append-metadata
   SUPPORTED_DEVICES += mr18
 endef