common: build ymodem only on need

Build ymodem only on need to shrink spl image size.

Signed-off-by: Jun Nie <jun.nie@linaro.org>

common/Makefile

@@ -121,6 +121,7 @@ obj-y += command.o
 obj-$(CONFIG_$(SPL_)LOG) += log.o
 obj-$(CONFIG_$(SPL_)LOG_CONSOLE) += log_console.o
 obj-y += s_record.o
-obj-y += xyzModem.o
+obj-$(CONFIG_CMD_LOADB) += xyzModem.o
+obj-$(CONFIG_$(SPL_TPL_)YMODEM_SUPPORT) += xyzModem.o
 
 obj-$(CONFIG_AVB_VERIFY) += avb_verify.o