src/cyw43_ll: Add event hooks during firmware load.

Loading the WiFi firmware can cause delays on startup which causes problems
for tasks like USB that must be executed periodically.  Add some hooks to
allow such background processes to run.

See micropython/micropython#8904.

Fixes issue #11.

src/cyw43_config.h

@@ -102,3 +102,9 @@
 #ifndef CYW43_FAIL_FAST_CHECK
 #define CYW43_FAIL_FAST_CHECK(res) (res)
 #endif
+
+// This should be defined by the port if needed, to let background processes
+// run during long blocking operations such as WiFi initialisation.
+#ifndef CYW43_EVENT_POLL_HOOK
+#define CYW43_EVENT_POLL_HOOK
+#endif

src/cyw43_ll.c

@@ -466,6 +466,8 @@ static int cyw43_download_resource(cyw43_int_t *self, uint32_t addr, size_t raw_
     #endif
 
     for (size_t offset = 0; offset < len; offset += block_size) {
+        CYW43_EVENT_POLL_HOOK;
+
         size_t sz = block_size;
         if (offset + sz > len) {
             sz = len - offset;
@@ -1416,6 +1418,8 @@ static void cyw43_clm_load(cyw43_int_t *self, const uint8_t *clm_ptr, size_t clm
 
     const size_t clm_dload_chunk_len = CLM_CHUNK_LEN;
     for (size_t off = 0; off < clm_len; off += clm_dload_chunk_len) {
+        CYW43_EVENT_POLL_HOOK;
+
         uint32_t len = clm_dload_chunk_len;
         uint16_t flag = 1 << 12; // DLOAD_HANDLER_VER
         if (off == 0) {
@@ -1497,7 +1501,9 @@ int cyw43_ll_bus_init(cyw43_ll_t *self_in, const uint8_t *mac) {
         }
 
         cyw43_spi_gpio_setup();
+        CYW43_EVENT_POLL_HOOK;
         cyw43_spi_reset();
+        CYW43_EVENT_POLL_HOOK;
 
         // Check test register can be read
         for (int i = 0; i < 10; ++i) {