[Toybox] runcon requires CONFIG_TOYBOX_NORECURSE=y

[enh]

runcon works by saying "when i exec, switch to this context" and then
calling exec. unfortunately, xexec is too clever and will avoid
actually exec'ing if it recognizes that it would just be a call back
into toybox.

so "runcon blah /system/bin/id" would work, because toybox would
actually exec, but "runcon blah id" wouldn't, because toybox would
just toy_exec "id". this was subtle enough to go unnoticed for some
time.

i've "fixed" it for now by just setting CONFIG_TOYBOX_NORECURSE=y
(https://android-review.googlesource.com/#/c/196953/) but i wondered
whether we should have a #error or something in runcon.c to make it
clear if you're building a broken configuration?

-- 
Elliott Hughes - http://who/enh - http://jessies.org/~enh/
Android native code/tools questions? Mail me/drop by/add me as a reviewer.