Convert FreeRTOS-related libraries from interface to static, because
* other components in the Open IoT SDK are static libraries
* an interface library's source files are treated as source files of
libraries and executables that link it - if multiple of them link
against the same interface library, the files get compiled multiple
times and may result in linking errors due to multiple definitions of
symbols.

Signed-off-by: Lingkai Dong <lingkai.dong@arm.com>