buildroot

mtools: add support for building for target

This patch adds a menu option and updates the mtools.mk file to support
building mtools for the target.

[Peter: use same upstream URL as host variant]
Signed-off-by: Frank Hunleth <fhunleth@troodon-software.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>