NET_BUF_POOL_FIXED_DEFINE macro

@def NET_BUF_POOL_FIXED_DEFINE Define a new pool for buffers based on fixed-size data Defines a net_buf_pool struct and the necessary memory storage (array of structs) for the needed amount of buffers. After this, the buffers can be accessed from the pool through net_buf_alloc. The pool is defined as a static variable, so if it needs to be exported outside the current module this needs to happen with the help of a separate pointer rather than an extern declaration. The data payload of the buffers will be allocated from a byte array of fixed sized chunks. This kind of pool does not support blocking on the data allocation, so the timeout passed to net_buf_alloc will be always treated as K_NO_WAIT when trying to allocate the data. This means that allocation failures, i.e. NULL returns, must always be handled cleanly. If provided with a custom destroy callback, this callback is responsible for eventually calling net_buf_destroy() to complete the process of returning the buffer to the pool.