build: Create a home partition image

u-boot's fastboot implementation is unfortunately stupid enough that we can't
simply use "fastboot format:ext4" to create the filesystem for the home

Instead, we create the partition using Mendel's build system and use img2simg to
create a super tiny sparse image.

Change-Id: If53562b0aab6e448715dc02b128c6abeb158593a
2 files changed