rtos: use RTX identifier
rtos: use RTX identifier

os_thread_t and family are internal and should not be used (thus we included
internal header file). Instead, use those that are exposed via rtx_os.h file.

mbedmicro/mbed,c1728p9/mbed-os,c1728p9/mbed-os,mbedmicro/mbed,andcor02/mbed-os,mbedmicro/mbed,c1728p9/mbed-os,betzw/mbed-os,andcor02/mbed-os,betzw/mbed-os,kjbracey-arm/mbed,kjbracey-arm/mbed,betzw/mbed-os,mbedmicro/mbed,kjbracey-arm/mbed,c1728p9/mbed-os,andcor02/mbed-os,andcor02/mbed-os,andcor02/mbed-os,andcor02/mbed-os,betzw/mbed-os,c1728p9/mbed-os,kjbracey-arm/mbed,c1728p9/mbed-os,betzw/mbed-os,mbedmicro/mbed,betzw/mbed-os