migrate from req_ctx to msgb

We now generalize the USB communiction and abandon the 'req_ctx'
structure inherited from openpcd.  Instead we use the libosmocore 'msgb'
structure to handle incoming and outgoing USB tranfers.  We also use
linuxlist-based msgb-queues for each endpoint.

firmware/libcommon/include/usb_buf.h

@@ -0,0 +1,28 @@
+#pragma once
+
+#include "osmocom/core/linuxlist.h"
+#include "osmocom/core/msgb.h"
+
+/* buffered USB endpoint (with queue of msgb) */
+struct usb_buffered_ep {
+	/* endpoint number */
+	uint8_t ep;
+	/* OUT endpoint (1) or IN/IRQ (0)? */
+	uint8_t out_from_host;
+	/* currently any transfer in progress? */
+	volatile uint32_t in_progress;
+	/* Tx queue (IN) / Rx queue (OUT) */
+	struct llist_head queue;
+};
+
+struct msgb *usb_buf_alloc(uint8_t ep);
+void usb_buf_free(struct msgb *msg);
+int usb_buf_submit(struct msgb *msg);
+struct llist_head *usb_get_queue(uint8_t ep);
+int usb_drain_queue(uint8_t ep);
+
+void usb_buf_init(void);
+struct usb_buffered_ep *usb_get_buf_ep(uint8_t ep);
+
+int usb_refill_to_host(uint8_t ep);
+int usb_refill_from_host(uint8_t ep);