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).

to use old style of output add this to boardconfig of device tree:
TARGET_USES_BLOCK_BASED_OTA := false

Change-Id: I1dcde9c9a212c34108629537cb8c1088a0373573