tpm: Add a proper Kconfig option for crc8 in SPL

The current approach is a bit of a hack and only works for the tpm
subsystem. Add a Kconfig so that crc8 can be enabled in SPL for other
purposes.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/lib/Makefile b/lib/Makefile
index d77b33e..a282e40 100644
--- a/lib/Makefile
+++ b/lib/Makefile
@@ -57,12 +57,13 @@
 
 obj-$(CONFIG_$(SPL_TPL_)TPM) += tpm-common.o
 ifeq ($(CONFIG_$(SPL_TPL_)TPM),y)
-obj-y += crc8.o
 obj-$(CONFIG_TPM) += tpm_api.o
 obj-$(CONFIG_TPM_V1) += tpm-v1.o
 obj-$(CONFIG_TPM_V2) += tpm-v2.o
 endif
 
+obj-$(CONFIG_$(SPL_TPL_)CRC8) += crc8.o
+
 obj-y += crypto/
 
 obj-$(CONFIG_$(SPL_TPL_)GENERATE_ACPI_TABLE) += acpi/