Adding AVB-specific kernel patches for reference

Includes the needed kernel patches for AVB under the new folder contrib/* and
updates README.md for this info. Also adds a link for A/B related build system
variables.

The two patches are generated by `git format-patch` from android common
kernel 4.4:

    https://android.googlesource.com/kernel/common/+/android-4.4

The 0002-*.patch is generated by merging four commits:
    1. git rebase -i 8d6f006d608c3b03652fb919e496945f2d4d4f1d
    2. Removes unrelated commits, and s/pick/squash for the following
       commits:
       2.1 squash e5272d4c20b3 ANDROID: AVB: Only invalidate vbmeta when told to do so.
       2.2 squash 815c6db10b07 ANDROID: AVB: Fix invalidate_vbmeta_submit().
       2.3 squash f0893c7ddba2 ANDROID: AVB: Fix linter errors.
    3. Updates commit message.

Bug: None
Test: None

Change-Id: Icb007769db4b7454d98cd49a9827e0ad5820fd31