Fix Bug in genzehn on OSX

-Empty program options description made boost crash malloc
Add nspire-tools path command

-Returns absolute path to the ndless sdk
Fix Makefile.tpl

-$(DESTDIR) used together with vpath
Newer version of nspireio
"install" target for luna

-Installs in /usr/bin if run by root, else ~/bin
Minor bugfixes

-Use c++-style cast for SCREEN_BASE_ADDRESS in libndls.h if possible
-Add description to program option list to genzehn.cpp to prevent crashes on OSX
Add support for --gc-sections