Generate flatten images if kernel_images have
build_system_dlkm_flatten is set True. These
images don't have the `uname -r` in the directory
structure and all modules are available as
lib/modules/*.ko with modules.load and modules.dep
depmod artifacts.

Generates images as follow:

If system_dlkm_fs_type is set:
  system_dlkm.flatten.<system_dlkm_fs_type>.img

If system_dlkm_fs_types is set:
  system_dlkm.flatten.<fs_type>.img for each fs types

By default when no fs type is specified:
  system_dlkm.flatten.ext4.img

Bug: 301178664
Test: Manual image verification with different options
Test: TH
Change-Id: I4b7815d131d3d26adaa35c4550e76db71aa33936
Signed-off-by: Ramji Jiyani <ramjiyani@google.com>