Revert "core/makefile: User configurable block on/off"

how-to:
export USE_BLOCK=1

Change-Id: I06f889b286aab07eb6171d8546ff0d242076044e (reverted from commit 8871bf6fb811be0b4343a4add31f8c23284e3490)
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