Use cyw43_malloc/free macro instead of malloc/free to allow alternative implementations

[peterharperuk]

Bluetooth needs a tempoarary buffer to load firmware. Micropython won't work safely with malloc and free so allow these calls to be changed by using cyw43_alloc / free macros.

[peterharperuk]

This MP change prevents us using malloc and free micropython/micropython@c80e7c1

[dpgeorge]

This MP change prevents us using malloc and free

Oh yes! Before making that change I did check to see if pico-sdk used malloc/free at all and came to the conclusion that it was not needed (I looked through the firmware.elf file of MicroPython). But I did not check with BT enabled...

An alternative is that in MicroPython we could leave a little space for the C heap. The downside with that approach is that C heap will be mostly unused and hence wasted. So adding a cyw43_malloc hook seems like a better approach.

Another alternative: add an argument to cyw43_btbus_init() which is a temporary buffer that the caller must supply. And also a constant (eg #define CYW43_BT_BUF_SIZE 1024) so the caller knows how much buffer space needs to be passed in. Then the caller (eg MicroPython bindings) can allocate this, eg statically, on the C stack, on the GC heap.

[peterharperuk]

Updated to rename cyw43_alloc to cyw43_malloc