Export HOME in pmb.chroot.root() and -.user()

Set HOME to /root for commands started with pmb.chroot.root() and to
/home/pmos for commands started with pmb.chroot.user().

POSIX requires this variable to be set, see:
<http://pubs.opengroup.org/onlinepubs/009695399/basedefs/xbd_chap08.html>

And this prevents a crash in "adb", which takes and alternative code
path if HOME is not set, that does not work with musl (fixes #1638).
Thanks to @ryang2678 for figuring this out!

pmb/chroot/root.py

@@ -70,6 +70,7 @@ def root(args, cmd, suffix="native", working_dir="/", output="log",
     # Merge env with defaults into env_all
     env_all = {"CHARSET": "UTF-8",
                "HISTFILE": "~/.ash_history",
+               "HOME": "/root",
                "PATH": pmb.config.chroot_path,
                "SHELL": "/bin/ash",
                "TERM": "xterm"}

pmb/chroot/user.py

@@ -34,6 +34,9 @@ def user(args, cmd, suffix="native", working_dir="/", output="log",
     See pmb.helpers.run_core.core() for a detailed description of all other
     arguments and the return value.
     """
+    if "HOME" not in env:
+        env["HOME"] = "/home/pmos"
+
     flat_cmd = pmb.helpers.run.flat_cmd(cmd, env=env)
     cmd = ["busybox", "su", "pmos", "-c", flat_cmd]
     return pmb.chroot.root(args, cmd, suffix, working_dir, output,