Update Makefile to build libcore

Previously the makefile had libcore.rlib as a dependency for the
libcore target but had no rule which indicated how to actually build
libcore.rlib.  I have added that rule.
Minor README updates

I added link to Rust install site and showed that frdm-kl24z-rust.git
should be installed as a sibling to the main rust git repository.
Merge pull request #1 from adamgreen/master

Minor updates to Makefile and README