package/libopenh264: new package

[Thomas:
  - make the license info more specific: it's BSD-2c, not just BSD.
  - use a single conditional block to test the architecture, instead
    of multiple separate conditions.
  - add missing arm/armeb handling in the architecture condition.
  - make the host-nasm dependency only used on x86/x86-64, since it's
    not used for other architectures
  - group the ARCH= and ENABLE64BIT= make variable definitions in a
    LIBOPENH264_MAKE_OPTS variable, and use them at install time (in
    addition to build time) to avoid build issues.
  - wrap too long lines in the .mk file.]

Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>