generic: backport BLOCK OF support patch

generic: backport BLOCK OF support patch

Backport BLOCK OF support patch merged upstream and refresh pending
BLOCK patches.

This is a new way to declare partition table for BLOCK device (eMMC
currently supported) with the use of DTS.

Current pending patch are adapted to not cause regression with current
downstream implementation of a similar functionality.

Also enable the new OF_PARTITION config by default.

Link: https://github.com/openwrt/openwrt/pull/16663
Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>