liblp: Add lpmodify for target use

Primarily intended as test util to add/remove logical
partitions during tests.

Bug: 413763600
Test: 1) lpmodify add /dev/block/by-name/super foobar
         google_dynamic_partitions_b --size 1073741824
      2) lpmodify remove /dev/block/by-name/super foobar
      3) lpdump after lpmodify add/remove to check
         partition is correctly setup.
      4) dmctl create-from-super correctly sets up LP

Change-Id: Ib8eda277e60481cecb9e328597cc07fc3c811eae