feat(epicardium): Introduce os/core.h header for OS definitions

Documentation/epicardium/internal.rst

Documentation/index.rst

epicardium/api/control.c

epicardium/api/genapi.py

epicardium/ble/ble.c

epicardium/ble/ble_adv.c

epicardium/ble/ble_attc.c

epicardium/ble/ble_main.c

epicardium/ble/bondings.c

epicardium/ble/epic_att_api.c

epicardium/ble/epic_ble_api.c

epicardium/ble/ess.c

epicardium/ble/filetransfer.c

epicardium/ble/hid.c

epicardium/ble/hid_work.c

epicardium/drivers/bhi.c

epicardium/drivers/bme680.c

epicardium/drivers/bsec.c

epicardium/drivers/buttons.c

epicardium/drivers/gpio.c