devshell.bbclass: Allow running devshell and fakeroot together

devshell.bbclass: Allow running devshell and fakeroot together

Currently its hard to run a devshell complete with fakeroot context. This
patch allows the fakeroot flag on the task to do this, as with any other
task. Since we may need to start X terminal applications, we need to
only start the fakeroot session on the final command, hence the hoops
this code jumps through.

As always with fakeroot, you can break out and run a command without
the fake permissions with syntax like "PSEUDO_UNLOAD=1 <command>"

[YOCTO #3374]

(From OE-Core rev: 0a2662a48eaf0487db043c348e2834bb9cdd0466)

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
Signed-off-by: Saul Wold <sgw@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>