package/mono: fixing wrong monolite library path

Due to a bug in makefile logic, monolite libraries path will not
be selected when EXTERNAL_MCS parameter is specified.
Setting EXTERNAL_MCS to a non existent binary, makes the internal logic
to fallback to monolite, setting correctly both binary and
library paths.
This has the benefit to force host-mono to always use monolite as
bootstrap compiler also in case a mono compiler is already installed
on the system.

Fixes:
http://autobuild.buildroot.net/results/fc00952bef03cf3b4785be1a0d9c08dc84aa3f54/



[Peter: add comment explaining why this is done]
Signed-off-by: Angelo Compagnucci <angelo.compagnucci@gmail.com>
Reported-by: Rainer Berns <rainer.berns@berns-online.de>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>