Upstream commit/repo
  1e921369b2c92bb219fcef84a37d4d2347794c0f
  https://github.com/amachronic/microtar

Summary of Rockbox-specific changes:

- microtar.make, SOURCES
  Target build system integration.

- microtar-rockbox.c, microtar-rockbox.h
  Implements mtar ops with Rockbox filesystem API.

- microtar.c
  Define strncpy since Rockbox's libc doesn't define it.