avb2.0: add Android Verified Boot 2.0 library

Add libavb lib (3rd party library from AOSP), that implements support of
AVB 2.0. This library is used for integrity checking of Android partitions
on eMMC.

libavb was added as it is and minimal changes were introduced to reduce
maintenance cost, because it will be deviated from AOSP upstream in the future.

Changes:
- license headers changed to conform SPDX-style
- avb_crc32.c dropped
- updates in avb_sysdeps_posix.c/avb_sysdeps.h

For additional details check [1] AVB 2.0 README.

[1] https://android.googlesource.com/platform/external/avb/+/master/README.md



Signed-off-by: Igor Opaniuk <igor.opaniuk@linaro.org>