Hook incremental ext4 image generation into build

Allows passing the *.base_fs or *.map files for system and vendor in
INTERNAL_SYSTEM_BASE_FS_PATH and INTERNAL_VENDOR_BASE_FS_PATH variables
respectively.

Internal Design Doc: go/incremental-ext4
BUG: 26839493
Signed-off-by: Mohamad Ayyash <mkayyash@google.com>

Change-Id: Ie57ef6dbfa6084268b6535fe0a673a3b4aaa6e2f
Signed-off-by: Mohamad Ayyash <mkayyash@google.com>