Add avbtool for working with Android Verified Boot

It's also an additional requirement to use the (already deprecated?!)
GKI 2.0 signing arguments[1] with mkbootimg.

[1] https://android.googlesource.com/platform/system/tools/mkbootimg/+/339274e/mkbootimg.py#555

README.md

@@ -28,7 +28,7 @@ Currently the following tools are supported:
 * mke2fs.android (required by fastboot)
 * simg2img, img2simg, append2simg
 * lpdump, lpmake, lpadd, lpflash, lpunpack
-* mkbootimg, unpack_bootimg, repack_bootimg
+* mkbootimg, unpack_bootimg, repack_bootimg, avbtool
 
 The build system itself works quite well and is already being used for
 the Alpine Linux [android-tools package][alpine-linux] which I maintain.

vendor/CMakeLists.avb.txt

@@ -0,0 +1 @@
+install(PROGRAMS avb/avbtool.py DESTINATION bin RENAME avbtool)

vendor/CMakeLists.txt

@@ -71,6 +71,7 @@ include(CMakeLists.fastboot.txt)
 include(CMakeLists.mke2fs.txt)
 include(CMakeLists.partition.txt)
 include(CMakeLists.mkbootimg.txt)
+include(CMakeLists.avb.txt)
 
 # Targets which should be installed by `make install`.
 install(TARGETS