forward declare board_millis for OPT_OS_CUSTOM

[duckylotl]

When porting examples to a custom OS the missing forward declaration of board_millis creates problems.
The forward declaration lets custom OS ports use the board_api.h for use of board_usb_get_serial and implement board_millis anywhere if needed with the correct signature.

The CDC examples include the bsp/board_api.h in usb_descriptors.c for use of board_usb_get_serial. board_api.h also includes implementation of board_delay which uses board_millis. But in case of OPT_OS_CUSTOM it has not been declared anywhere, so it gets implicitly declared as int board_millis(void) which will not link with an implementation using the correct unsigned int return type.

[hathach]

Look good, thank you.

note: ci failed due to my incorrect labeler workflow and can be safe to ignore