buildroot

libamcodec: new package

Signed-off-by: Dagg Stompler <daggs@gmx.com>
[Thomas:
 - use the AML_LIBS_STAGING_DIR variable instead of PREFIX/M_PREFIX to
   indicate the compiler sysroot, which allows to use a foreach loop
   in the build commands.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>