Anbox support

Created by: PureTryOut

Back in August, @MartijnBraam started packaging Anbox but got stuck somewhere in the process. I just rebased the branch to the latest master and updated some dependencies.

However one of it's dependencies, dbus-cpp, fails to compile (we package it ourselves). We're guessing it's a musl problem, which might make this quite hard to resolve.

[  4%] Building CXX object src/core/dbus/CMakeFiles/dbus-cpp.dir/fixture.cpp.o
/home/pmos/build/src/build/src/core/dbus/fixture.cpp:26:29: fatal error: core/posix/exec.h: No such file or directory
 #include <core/posix/exec.h>
                             ^
compilation terminated.
make[2]: *** [src/core/dbus/CMakeFiles/dbus-cpp.dir/build.make:63: src/core/dbus/CMakeFiles/dbus-cpp.dir/fixture.cpp.o] Error 1
make[1]: *** [CMakeFiles/Makefile2:1451: src/core/dbus/CMakeFiles/dbus-cpp.dir/all] Error 2
make: *** [Makefile:141: all] Error 2