how to write a board support package
A board support package is the layer that sits directly above the hardware and below the RTOS kernel. It provides as abstraction from the hardware. Writing a BSP is different for different RTOSs. However, it will almost always involve initial bootup, hardware initialization and then placing a call to the kernel inititalization and spawning the root task. The BSP is written according to the hardware present on the board.
For your case, you need the basic bootup code which would be in assembly. This code can either be found from Net (with luck) or may need to be written (by reading the microprocessor manual). This code should perform minimal bootup (disable interrupts, initialize RAM and other basic tasks as specified by the manual) and then call C routine to carry forward. We call C routine to make it easy to write code. It may not be necessary. Check with your particular RTOS BSP Developer manual to see if it is required.
From this point onwards, its all C code to initialize all the hardware and then initialize the Kernel.