[][Refactor eMMC image of MT7981]

Refactor eMMC image of MT7981.
The whole image format is changed:
 - Kernel and rootfs reside in individual partitions.
 - rootfs_data split from rootfs partition using loop device with offset.
 - Upgrading firmware usig tarball with saving config files supported.
 - No recovery partition needed.
 - No GPT partition offset and size restricted.

Important note:
Please select the following packages to make emmc firmware upgrading work:
e2fsprogs mkf2fs blkid blockdev losetup kmod-fs-ext4 kmod-fs-f2fs


Change-Id: I17ad85efa235d068504eeb848431b0c73ae768b4

Change-Id: Iae56e7776b049d3fd9f79861523379794e99c7f0
Reviewed-on: https://gerrit.mediatek.inc/c/openwrt/feeds/mtk_openwrt_feeds/+/5374466
5 files changed