lib: add crc16.o if CONFIG_MMC_SPI_CRC_ON for SPL build

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.
diff --git a/lib/Makefile b/lib/Makefile
index 8ba745f..2d29cda 100644
--- a/lib/Makefile
+++ b/lib/Makefile
@@ -88,6 +88,7 @@
 ifdef CONFIG_SPL_BUILD
 obj-$(CONFIG_SPL_YMODEM_SUPPORT) += crc16.o
 obj-$(CONFIG_$(SPL_TPL_)HASH_SUPPORT) += crc16.o
+obj-$(CONFIG_MMC_SPI_CRC_ON) += crc16.o
 obj-y += net_utils.o
 endif
 obj-$(CONFIG_ADDR_MAP) += addr_map.o