liblp: Reserve the first logical block of the super partition.

Traditionally the first 512 bytes of a partition can be interpreted as
an MBR. To prevent any compatibility issues, we explicitly zero the
first 4096 bytes of the super partition (one logical block, on most
systems).

Bug: 116802789
Test: liblp_test gtest
      device with super partition flashes and boots

Change-Id: I29688ca75dbb52442f1464e8ab35893678a4f79e