src/your-source.c

/**
 * Depending on the type of package, there are different
 * compilation rules for this directory.  This comment applies
 * to packages of type "pkg."  For other types of packages,
 * please view the documentation at http://mynewt.apache.org/.
 *
 * Put source files in this directory.  All files that have a *.c
 * ending are recursively compiled in the src/ directory and its
 * descendants.  The exception here is the arch/ directory, which
 * is ignored in the default compilation.
 *
 * The arch/<your-arch>/ directories are manually added and
 * recursively compiled for all files that end with either *.c
 * or *.a.  Any directories in arch/ that don't match the
 * architecture being compiled are not compiled.
 *
 * Architecture is set by the BSP/MCU combination.
 */