A usbdev has only one serial string, so use a unique macro to control it.
For boards that enable board serial string using COMPOSITE_BOARD_SERIALSTR,
PL2303_BOARD_SERIALSTR, CDCACM_BOARD_SERIALSTR, USBADB_BOARD_SERIALSTR,
USBMSC_BOARD_SERIALSTR, and RNDIS_BOARD_SERIALSTR, they need to be replaced
with BOARD_USBDEV_SERIALSTR.
Signed-off-by: zhangyuan21 <zhangyuan21@xiaomi.com>