By default the SDIO (F4) or SDMMC1 (L4, F7) is used as the SD card
peripheral, but if a board config defines MICROPY_HW_SDMMC2_CK and other
pins then the SD card driver will use SDMMC2.
Build happens in 3 stages:
1. Zephyr config header and make vars are generated from prj.conf.
2. libmicropython is built using them.
3. Zephyr is built and final link happens.
Allows to get event time for a head item in the queue. The usecase
if waiting for the next event *OR* I/O completion. I/O completion may
happen before event triggers, and then wait should continue for the
remaining event time (or I/O completion may schedule another earlier
event altogether).
The new function has a strongly provisional status - it may be converted
to e.g. peek() function returning all of the event fields, not just time.
With the existing timeout of 100ms the transfer would end prematurely if
the baudrate was low and the number of bytes to send was high. This patch
fixes the problem by making the timeout proportional to the number of bytes
that are being transferred.