ath79: meraki-mr18: fix initramfs build

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: 1d49310fdb5e ("ath79: add Cisco Meraki MR18")
Signed-off-by: Lech Perczak <lech.perczak@gmail.com>
Link: https://github.com/openwrt/openwrt/pull/17680
Signed-off-by: Robert Marko <robimarko@gmail.com>
(cherry picked from commit 7423e67e1b8c7e4c9f527d1300dbeaab72b1f95f)