build_image: add support for specifying number of inodes

Add support for specifying number of inodes when creating
system, vendor, oem partitions.  These are all read-only
and have no use for extra inodes.  Removing extra inodes
saves a lot of space.

Bug: 32246383
Change-Id: I13f1d4614b64a4abc752c42a1c65d3d151481c21
(cherry picked from commit b59eca35)