Summary:
The configs all need default values which targets then override. This
one was an empty string which made the logic report an error. The only
reason it wasn't a build failure was because of a stray `:`.
Few printf config options have been setup using this new config system
along with their baremetal overrides. A follow up patch will add generation
of doc/config.rst, which will contain the full list of libc config options
and short description explaining how they affect the libc.
Reviewed By: gchatelet
Differential Revision: https://reviews.llvm.org/D159158