Stop passing `--block` and MKBOOTIMG to ota_from_target_files.

- `--block` has been a no-op since Oreo [1] -- ota_from_target_files
  will always build block-based OTAs or A/B OTAs.
- MKBOOTIMG is no longer needed when calling ota_from_target_files, at
  least for the callsite in Makefile. Because target_files.zip has been
  including boot/recovery images since Lollipop [2].

This CL also switches to long options to improve readability.

[1] commit 457cbf6a
[2] commit 3c84f569

Test: TreeHugger
Change-Id: I801bd0525d2529156bbf783698661d90ba24017a