core: Allow to disable block based OTA's

on Xiaomi Mi2, it causes that the user can't install GAPPS because
the filesystem is smaller than the partition then(users resize the syspart).

Change-Id: I1dcde9c9a212c34108629537cb8c1088a0373573
Signed-off-by: cj360 <ayunker551@gmail.com>

Conflicts:
	core/Makefile