Add BOARD_CUSTOM_BOOTIMG_MK support

This is a combination of 2 commits.
This is the 1st commit message:

    Add BOARD_CUSTOM_BOOTIMG_MK support

    Simplified version of the previous implementation. Recovery's ramdisk
    is spun off from the main recovery target again to allow overriding
    just the image-generation step

    [mikeioannina]: Squash cm-13.0 changes and adapt to N

    Change-Id: I058d214f0cf2d05b7621b369ef1f8a983c3ac258

This is the commit message #2:

    build: Handle custom boot images properly

    When a pre-built image should be used, it should be stored in the
    target files zip so that it can be used with external signing
    processes.

    Original-Change-Id: I2661af9ac58af30bb9314b552775046d3abf44e0
    Change-Id: I10b3bd0bb33489b8ffb26d16d002f8dd6ff405ad

[aleasto]
  Rewritten for R, where the recovery ramdisk is created via
  make dependencies, rather than $(call)s

Change-Id: I058d214f0cf2d05b7621b369ef1f8a983c3ac258