boot: serial: Adapt to Zephyr's new CRC APIs

As part of https://github.com/zephyrproject-rtos/zephyr/pull/42457,
Zephyr recently changed its CRC APIs. The commit below
adapted the use of mcumgr in-tree, but MCUboot was missing the change:
https://github.com/zephyrproject-rtos/zephyr/commit/07c78e515c827c8346a758a5ce04cd56a46ae124

Note that although on other platforms the function called is
crc_ccitt(), the CRC16 actually used by MCUboot/mcumgr is:
https://reveng.sourceforge.io/crc-catalogue/16.htm#crc.cat.crc-16-xmodem



Signed-off-by: Carles Cufi <carles.cufi@nordicsemi.no>