CONFIG_MMC_SPI_CRC_ON needs the crc16 functions, but it was not included
in an SPL build.  For non-SPL builds, crc16.o is already added
unconditionally.  This also removes CONFIG_SPL_YMODEM_SUPPORT from the
sifive board configs, which is only relevant for some ARM boards and was
only set for its side effect of adding crc16.o.