Don't use block mode by default, allow it to be chosen

Currently this uses the same logic as backuptool (but inverted)

i.e. block mode will be disabled in all cases where backuptool is enabled
which is the current default for all Omni builds.

Block mode is not needed now, and may never be needed for custom projects,
as block mode is preparation for dm-verity, which is mostly pointless on
devices with unlocked bootloaders since the kernel already has broken
the chain of trust.

TODO:  Determine if Nexus 9 *requires* blockmode.  If it does, this
may need to be reworked to allow the device to specify blockmode.

Change-Id: I36725b44f4066a43ec7e1615bd4b072f7eb3db28
blockmode: allow it to be defined per device

 - default: blockmode is set to false
 - to enable place TARGET_OTA_USE_BLOCKMODE := true in the device BoardConfig.mk

Change-Id: I15eb5c2e0af98517c98deada612d6236047ed5fb