Put rust src dir into SRC_DIR
Make SRC_DIR overridable
Put distribution dir into DIST_DIR
Put target strings into TARGET and TOOLCHAIN_TARGET
Patch CROSS_PREFIX in rust's platform.mk too

In a parallel build the build system may use the default prefix from
platform.mk which might not match the actual toolchain
Merge pull request #13 from gkoz/add_vars

Put some moving parts into variables